drm/amd/display: Prune Invalid Modes For HDMI Output

[Why]
1. HDMI does not have 6 bpc support. Having 6 bpc pass validation
does not comply with spec.
2. Validate 420 only for native HDMI, but not apply to pcon use
case.
3. Current mode validation log is not readable.

[how]
1. Cap 8 bpc for dp-hdmi converter.
2. Validate yuv420 for pcon use case as well,
   if rgb/yuv444 8bpc cannot fit into pcon bw limitation of
   the link from the converter to HDMI sink.
3. Add readable pixel_format and color_depth into debug log.

Reviewed-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Signed-off-by: Zaeem Mohamed <zaeem.mohamed@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>