drm/format-helper: Remove unnecessary conversion helpers

Drivers only emulate XRGB8888 framebuffers. Remove all conversion
helpers that do not use XRGB8888 as their source format. Also remove
some special cases for alpha formats in the blit helper.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230102112927.26565-14-tzimmermann@suse.de