drm: add drm_format_helper.c to kerneldoc

Also drop the dstclip parameter sphinx has warned about (leftover from
an earlier patch version).

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20190416090533.28374-1-kraxel@redhat.com