drm/rcar-du: Set GEM CMA functions with DRM_GEM_CMA_DRIVER_OPS_WITH_DUMB_CREATE

DRM_GEM_CMA_DRIVER_OPS_WITH_DUMB_CREATE sets the functions in
struct drm_driver to their defaults. No functional changes are
made.

v2:
	* update for DRM_GEM_CMA_DRIVER_OPS_WITH_DUMB_CREATE

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605073247.4057-31-tzimmermann@suse.de