drm/driver: Add defaults for .gem_prime_export/import callbacks

The majority of drivers use drm_gem_prime_export() and
drm_gem_prime_import() for these callbacks so let's make them the
default.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181110145647.17580-2-noralf@tronnes.org

include/drm/drm_drv.h

@@ -471,6 +471,8 @@ struct drm_driver {
 	 * @gem_prime_export:
 	 *
 	 * export GEM -> dmabuf
+	 *
+	 * This defaults to drm_gem_prime_export() if not set.
 	 */
 	struct dma_buf * (*gem_prime_export)(struct drm_device *dev,
 				struct drm_gem_object *obj, int flags);
@@ -478,6 +480,8 @@ struct drm_driver {
 	 * @gem_prime_import:
 	 *
 	 * import dmabuf -> GEM
+	 *
+	 * This defaults to drm_gem_prime_import() if not set.
 	 */
 	struct drm_gem_object * (*gem_prime_import)(struct drm_device *dev,
 				struct dma_buf *dma_buf);