drm/vkms: Create KUnit tests for YUV conversions

drm/vkms: Create KUnit tests for YUV conversions

Create KUnit tests to test the conversion between YUV and RGB. Test each
conversion and range combination with some common colors.

The code used to compute the expected result can be found in comment.

[Louis Chauvet:
- fix minor formating issues (whitespace, double line)
- change expected alpha from 0x0000 to 0xffff
- adapt to the new get_conversion_matrix usage
- apply the changes from Arthur
- move struct pixel_yuv_u8 to the test itself]

Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20250415-yuv-v18-6-f2918f71ec4b@bootlin.com
Signed-off-by: Louis Chauvet <louis.chauvet@bootlin.com>