drm/format-helper: Print warning on missing format conversion

Not all possible format conversions are supported yet. Print a
warning on unsupported combinations.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220425075939.30450-2-tzimmermann@suse.de