drm/format-helper: Rename test cases to make them more generic

The tests available at the moment only check the conversion from
XRGB8888 to RGB332. However, more conversions will be tested in the
future.

In order to make the struct and functions present in the tests more
generic, rename xrgb8888_to_rgb332_* to convert_xrgb8888_*.

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: https://patchwork.freedesktop.org/patch/msgid/20220726230916.390575-3-jose.exposito89@gmail.com