drm/stm: 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>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605073247.4057-35-tzimmermann@suse.de