drm/amd/display: Add fallback path for YCBCR422

drm/amd/display: Add fallback path for YCBCR422

[Why]
DP validation may fail with multiple displays and higher color depths.
The sink may support others though.

[How]
When DP bandwidth validation fails, progressively fallback through:
- YUV422 8bpc (bandwidth efficient)
- YUV422 6bpc (reduced color depth)
- YUV420 (last resort)

This resolves cases where displays would show no image due to insufficient
DP link bandwidth for the requested RGB mode.

Suggested-by: Mauri Carvalho <mcarvalho3@lenovo.com>
Reviewed-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Mario Limonciello <Mario.Limonciello@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>