drm/amd/display: Test format info and blending

drm/amd/display: Test format info and blending

[WHAT]
Add KUnit coverage for amdgpu_dm_plane_get_format_info(), the overlay
universal-plane path of get_plane_formats(), the D-swizzle rejection
path of format_mod_supported(), and the DCN4.2 variant of
fill_blending_from_plane_state().

These exercise format-query and blending code paths that the existing
suite did not cover.

Assisted-by: Copilot:Claude-Opus-4.8 GPT-5.5
Reviewed-by: Bhawanpreet Lakha <bhawanpreet.lakha@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: George Zhang <george.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>