drm/amd/display: Add KUnit test for color helpers

drm/amd/display: Add KUnit test for color helpers

Add KUnit tests for six pure-logic functions in
amdgpu_dm_color.c: amdgpu_dm_fixpt_from_s3132,
__is_lut_linear, __drm_ctm_to_dc_matrix,
__drm_ctm_3x4_to_dc_matrix, amdgpu_tf_to_dc_tf,
and amdgpu_colorop_tf_to_dc_tf.

Expose these static functions under CONFIG_DRM_AMD_DC_KUNIT_TEST
and add a new amdgpu_dm_color.h header with the KUnit-only
prototypes. The test file re-declares the dc and amdgpu
transfer function enums locally to avoid pulling in the full
DC/amdgpu include chain that fails under UML.

26 test cases cover signed-magnitude to two's complement
conversion, LUT linearity detection, CTM-to-DC matrix
conversion, and transfer function enum mapping.

Assisted-by: Copilot:Claude-Opus-4.6
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>