drm/amd/display: Add debug option to force 1-tap chroma subsampling

[Why]
Default driver behaviour is 3-tap subsampling, so we should keep
it the same for test patterns as well. However, it is also useful
to force 1-tap subsampling for testing purposes.

Reviewed-by: Michael Strauss <michael.strauss@amd.com>
Acked-by: Hamza Mahfooz <hamza.mahfooz@amd.com>
Signed-off-by: George Shen <george.shen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>