drm: add drm_fb_memcpy_dstclip() helper

It is a drm_fb_memcpy() variant which checks the clip rectangle for the
destination too.

Common code between drm_fb_memcpy() and drm_fb_memcpy_dstclip() was
factored out into the drm_fb_memcpy_lines() helper function.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20190405095219.9231-3-kraxel@redhat.com