drm/format-helper: Flip src/dst-format branches in blit helper

Upcoming changes to the format conversion will mostly blit from
XRGB8888 to some other format. So put the source format in blit's
outer branches to make the code more readable. For cases where
a format only changes its endianness, such as XRGB565, introduce
dedicated branches that handle this for all formats.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230102112927.26565-7-tzimmermann@suse.de