drm/amd/display: Check link_index before accessing dc->links[]

{
                    "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/core/dc_link_exports.c"
                    ],
                    "versions": [
                        {
                            "version": "4562236b3bc0a28aeb6ee93b2d8a849a4c4e1c7c",
                            "lessThan": "8ad4c2cf6c1af18f556e5c591fb313b2982ddf60",
                            "status": "affected",
                            "versionType": "git"
                        },
                        {
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                            "lessThan": "032c5407a608ac3b2a98bf4fbda27d12c20c5887",
                            "status": "affected",
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                        {
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                        {
                            "version": "4562236b3bc0a28aeb6ee93b2d8a849a4c4e1c7c",
                            "lessThan": "8aa2864044b9d13e95fe224f32e808afbf79ecdf",
                            "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/core/dc_link_exports.c"
                    ],
                    "versions": [
                        {
                            "version": "4.15",
                            "status": "affected"
                        },
                        {
                            "version": "0",
                            "lessThan": "4.15",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.1.191",
                            "lessThanOrEqual": "6.1.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.6.87",
                            "lessThanOrEqual": "6.6.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.10.9",
                            "lessThanOrEqual": "6.10.*",
                            "status": "unaffected",
                            "versionType": "semver"
                        },
                        {
                            "version": "6.11",
                            "lessThanOrEqual": "*",
                            "status": "unaffected",
                            "versionType": "original_commit_for_fix"
                        }
                    ]
                }