drm/format-helper: Pass destination pitch to drm_fb_memcpy_dstclip()

The memcpy's destination buffer might have a different pitch than the
source. Support different pitches as function argument.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Maxime Ripard <maxime@cerno.tech>
Tested-by: nerdopolis <bluescreen_avenger@verizon.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210430105840.30515-2-tzimmermann@suse.de