drm/imx/dcss: Initialize DRM driver instance with CMA helper macro

The i.MX DCSS 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, .gem_prime_export and .gem_prime_import, were initialized
to their default implementations, so they are just kept empty now.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Laurentiu Palcu <laurentiu.palcu@oss.nxp.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200923102159.24084-8-tzimmermann@suse.de