drm/rcar-du: Use GEM CMA object functions

Create GEM objects with drm_gem_cma_create_object_default_funcs(), which
allocates the object and sets CMA's default object functions. Corresponding
callbacks in struct drm_driver are cleared. No functional changes are made.

Driver and object-function instances use the same callback functions, with
the exception of vunmap. The implementation of vunmap is empty and left out
in CMA's default object functions.

v3:
	* convert to DRIVER_OPS macro in a separate patch

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605073247.4057-30-tzimmermann@suse.de