ipu_planes_assign_pre() prototype is in "imx-drm.h" header file, so
include it to fix the following sparse warning:

drivers/gpu/drm/imx/ipuv3-plane.c:729:5: warning: symbol 'ipu_planes_assign_pre' was not declared. Should it be static?
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>