drm/amd/display: Add debugfs entry to force YUV420 output

[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>