drm/aspeed: Set driver CMA functions with DRM_GEM_CMA_DRIVER_OPS

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

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Acked-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>
Link: https://patchwork.freedesktop.org/patch/msgid/20200603083132.4610-5-tzimmermann@suse.de