drm/format-helper: Export drm_fb_clip_offset()

Provide a function that computes the offset into a blit destination
buffer. This will allow to move destination-buffer clipping into the
format-helper callers.

v4:
	* add missing '@' for parameter documentation
	* fix typo 'frambuffer'
v2:
	* provide documentation (Sam)
	* return 'unsigned int' (Sam, Noralf)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211110103702.374-2-tzimmermann@suse.de