drm/format-helper: Add KUnit tests for drm_fb_xrgb8888_to_rgb565()

Extend the existing test cases to test the conversion from XRGB8888 to
RGB565.

The documentation and the color picker available on [1] are useful
resources to understand this patch and validate the values returned by
the conversion function.

Tested-by: Tales L. Aparecida <tales.aparecida@gmail.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Link: http://www.barth-dev.de/online/rgb565-color-picker/ # [1]
Link: https://patchwork.freedesktop.org/patch/msgid/20220726230916.390575-5-jose.exposito89@gmail.com