drm/amd/display: Fix 'panel_cntl' could be null in 'dcn21_set_backlight_level()'

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "unaffected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/gpu/drm/amd/display/dc/hwss/dcn21/dcn21_hwseq.c"
                    ],
                    "versions": [
                        {
                            "version": "474ac4a875ca6fea3fc5183d3ad22ef7523dca53",
                            "lessThan": "94a506cbfd1bd4817502bed097e24cbab23cfce3",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "474ac4a875ca6fea3fc5183d3ad22ef7523dca53",
                            "lessThan": "2e150ccea13129eb048679114808eb9770443e4d",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "474ac4a875ca6fea3fc5183d3ad22ef7523dca53",
                            "lessThan": "0c863cab0e9173f8b6c7bc328bee3b8625f131b5",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
                            "version": "474ac4a875ca6fea3fc5183d3ad22ef7523dca53",
                            "lessThan": "e96fddb32931d007db12b1fce9b5e8e4c080401b",
                            "status": "affected",
                            "versionType": "git"
                        }
                    ]
                }

{
                    "product": "Linux",
                    "vendor": "Linux",
                    "defaultStatus": "affected",
                    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
                    "programFiles": [
                        "drivers/gpu/drm/amd/display/dc/hwss/dcn21/dcn21_hwseq.c"
                    ],
                    "versions": [
                        {
                            "version": "5.9",
                            "status": "affected"
                        },
                        {
                            "version": "0",
                            "lessThan": "5.9",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.1.191",
                            "lessThanOrEqual": "6.1.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.6.17",
                            "lessThanOrEqual": "6.6.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.7.5",
                            "lessThanOrEqual": "6.7.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.8",
                            "lessThanOrEqual": "*",
                            "status": "unaffected",
                            "versionType": "original_commit_for_fix"
                        }
                    ]
                }