drm/i915: introduce to_intel_bo helper

This is a purely cosmetic change to make changes in this area easier.
And hey, it's not only clearer and typechecked, but actually shorter,
too!

[anholt: To clarify, this is a change to let us later make
drm_i915_gem_object subclass drm_gem_object, instead of having
drm_gem_object have a pointer to i915's private data]

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
diff --git a/drivers/gpu/drm/i915/i915_gem_debug.c b/drivers/gpu/drm/i915/i915_gem_debug.c
index e602614..35507cf 100644
--- a/drivers/gpu/drm/i915/i915_gem_debug.c
+++ b/drivers/gpu/drm/i915/i915_gem_debug.c
@@ -72,7 +72,7 @@
 i915_gem_dump_object(struct drm_gem_object *obj, int len,
 		     const char *where, uint32_t mark)
 {
-	struct drm_i915_gem_object *obj_priv = obj->driver_private;
+	struct drm_i915_gem_object *obj_priv = to_intel_bo(obj);
 	int page;
 
 	DRM_INFO("%s: object at offset %08x\n", where, obj_priv->gtt_offset);
@@ -137,7 +137,7 @@
 i915_gem_object_check_coherency(struct drm_gem_object *obj, int handle)
 {
 	struct drm_device *dev = obj->dev;
-	struct drm_i915_gem_object *obj_priv = obj->driver_private;
+	struct drm_i915_gem_object *obj_priv = to_intel_bo(obj);
 	int page;
 	uint32_t *gtt_mapping;
 	uint32_t *backing_map = NULL;