drm/format-helper: Comment on RGB888 byte order

RGB888 is different than the other formats as most of its pixels are
unaligned and therefore helper functions do not use endianness conversion
helpers. Comment on this in the source code.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: José Expósito <jose.exposito89@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230102112927.26565-3-tzimmermann@suse.de