drm: Move dma functions into drm_legacy.h

Also drop the unneeded EXPORT_SYMBOL and sprinkle drm_legacy_ prefixes
where missing.

v2: Drop the confusing _core_ and drop extern, both suggested by
David.

Cc: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/drm_bufs.c b/drivers/gpu/drm/drm_bufs.c
index 9e04d6a..70ba89f 100644
--- a/drivers/gpu/drm/drm_bufs.c
+++ b/drivers/gpu/drm/drm_bufs.c
@@ -1338,7 +1338,7 @@
 				  task_pid_nr(current));
 			return -EINVAL;
 		}
-		drm_free_buffer(dev, buf);
+		drm_legacy_free_buffer(dev, buf);
 	}
 
 	return 0;