drm/xlnx: Initialize DRM driver instance with CMA helper macro

The xlnx driver uses CMA helpers with default callback functions.
Initialize the driver structure with the rsp CMA helper macro. The
driver is being converted to use GEM object functions as part of
this change.

Two callbacks, .dumb_destroy and .gem_prime_import, were initialized
to their default implementations, so they are just kept empty now.

v2:
	* initialize with DRM_GEM_CMA_DRIVER_OPS_WITH_DUMB_CREATE (Laurent)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hyun Kwon <hyun.kwon@xilinx.com>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200923102159.24084-22-tzimmermann@suse.de