[Why]
Even if YUV420 is available for video mode, YUV444 is still
automatically selected. This poses a problem for compliance tests.
[How]
Add a per-connector debugfs entry "force_yuv420_output" to force
selection of YUV420 mode.
Signed-off-by: Stylon Wang <stylon.wang@amd.com> Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com> Acked-by: Leo Li <sunpeng.li@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>