drm/format-helper: Add destination-buffer pitch to drm_fb_swab()

Add destination-buffer pitch as argument to drm_fb_swab(). Done for
consistency with the rest of the interface.

v2:
	* update documentation (Noralf)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20211110103702.374-4-tzimmermann@suse.de