drm: move ttm global code to core drm

I wrote this for the prime sharing work, but I also noticed other external
non-upstream drivers from a large company carrying a similiar patch, so I
may as well ship it in master.

Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/radeon.h b/drivers/gpu/drm/radeon/radeon.h
index 368fecf0..3cd1c47 100644
--- a/drivers/gpu/drm/radeon/radeon.h
+++ b/drivers/gpu/drm/radeon/radeon.h
@@ -235,7 +235,7 @@
  */
 struct radeon_mman {
 	struct ttm_bo_global_ref        bo_global_ref;
-	struct ttm_global_reference	mem_global_ref;
+	struct drm_global_reference	mem_global_ref;
 	struct ttm_bo_device		bdev;
 	bool				mem_global_referenced;
 	bool				initialized;