drm/format-helper: Move destination-buffer handling into internal helper

The format-convertion helpers handle several cases for different
values of destination buffer and pitch. Move that code into the
internal helper drm_fb_xfrm() and avoid quite a bit of duplication.

v2:
	* remove a duplicated blank line (Jose)
	* use drm_format_info_bpp() (Sam)
 	* fix vaddr_cached_hint bug (Sam)
	* add TODO on vaddr location (Sam)
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220808125406.20752-14-tzimmermann@suse.de