drm/format-helper: Add KUnit tests for drm_fb_xrgb8888_to_rgb888()

Extend the existing test cases to test the conversion from XRGB8888 to
RGB888.

Tested-by: Maíra Canal <mairacanal@riseup.net>
Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220926080837.65734-2-jose.exposito89@gmail.com