drm/format-helper: Share implementation among conversion helpers

Provide format-independent conversion helpers for system and I/O
memory. Implement most existing helpers on top of it. The source and
destination formats of each conversion is handled by a per-line
helper that is given to the generic implementation.

v2:
	* remove a blank line

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220427141409.22842-5-tzimmermann@suse.de