drm/tests: Add KUnit tests for drm_fb_swab()

Insert parameterized test for the drm_fb_swab() to ensure correctness
and prevent future regressions.

Each expected color has it bytes reversed in order, so xrgb would be
bgrx.

Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Reviewed-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20230814-gsoc-drm-format-test-v2-v3-2-bd3e9f9bc2fb@riseup.net