drm/amd/display: Add DSC check to seamless boot validation

[Why & How]
We want to immediately fail seamless boot validation if DSC is active,
as VBIOS currently does not support DSC timings. Add a check for
the relevant flag in dc_validate_seamless_boot_timing.

Signed-off-by: Anthony Wang <anthony1.wang@amd.com>
Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>