drm/amd/display: Add null check for pipe_ctx->plane_state in dcn20_program_pipe

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                        "drivers/gpu/drm/amd/display/dc/hwss/dcn20/dcn20_hwseq.c"
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{
                    "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/dcn20/dcn20_hwseq.c"
                    ],
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                            "version": "4.15",
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                        {
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