drm/amd/display: Clean up pixel format types

[Why & How]
Equate the first non-subsampled video surface format to
the enum SURFACE_PIXEL_FORMAT_SUBSAMPLE_END since it's
not a real format.

Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Aric Cyr <aric.cyr@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>