drm/format-helper: Remove optional byte-swap from line convertion

Implement per-pixel byte swapping in a separate conversion helper
for the single function that requires it. Select the correct helper
for each conversion.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220427141409.22842-3-tzimmermann@suse.de