drm/radeon/kms: add info query for backend map

The 3D driver need to get the pipe to backend
map to certain things.  Add a query to get the
info.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/radeon/rv770.c b/drivers/gpu/drm/radeon/rv770.c
index 4de5189..4720d00 100644
--- a/drivers/gpu/drm/radeon/rv770.c
+++ b/drivers/gpu/drm/radeon/rv770.c
@@ -778,6 +778,7 @@
 								(cc_rb_backend_disable >> 16));
 
 	rdev->config.rv770.tile_config = gb_tiling_config;
+	rdev->config.rv770.backend_map = backend_map;
 	gb_tiling_config |= BACKEND_MAP(backend_map);
 
 	WREG32(GB_TILING_CONFIG, gb_tiling_config);