drm/format-helper: Add Kunit tests for drm_fb_xrgb8888_to_mono()

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

Signed-off-by: Arthur Grillo <arthurgrillo@riseup.net>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230311125141.564801-2-arthurgrillo@riseup.net