drm/amd/display: Unify force_yuv debugfs into force_yuv_pixel_format

[Why]
The connector exposed a single force_yuv420_output boolean debugfs and
carried force_yuv420_output / force_yuv422_output boolean fields to force a
chroma encoding. This cannot express "force RGB" or "force YCbCr444", and
diverges from the upstream amdgpu_dm which uses a single
force_yuv_pixel_format field keyed on enum dc_pixel_encoding.

[How]
- Replace the two boolean fields with a single uint8_t
  force_yuv_pixel_format holding an enum dc_pixel_encoding value
  (PIXEL_ENCODING_UNDEFINED == no override).
- Replace the force_yuv420_output boolean debugfs with a read/write
  force_yuv_pixel_format file that takes the encoding directly
  (1=RGB, 2=YCbCr422, 3=YCbCr444, 4=YCbCr420), validated against
  PIXEL_ENCODING_COUNT.
- Convert the existing readers/writers in amdgpu_dm_connector.c to the
  new field, preserving current behaviour.
- Add YCbCr444 force support now that the field can express it.

v2: Merge with changed from drm-misc (Alex)

Assisted-by: Copilot:claude-opus-4.8
Reviewed-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Signed-off-by: Roman Li <roman.li@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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