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_debugfs.c b/drivers/gpu/drm/i915/i915_debugfs.c
index 1376dfe..bb3a4a8 100644
--- a/drivers/gpu/drm/i915/i915_debugfs.c
+++ b/drivers/gpu/drm/i915/i915_debugfs.c
@@ -225,7 +225,7 @@
 		} else {
 			struct drm_i915_gem_object *obj_priv;
 
-			obj_priv = obj->driver_private;
+			obj_priv = to_intel_bo(obj);
 			seq_printf(m, "Fenced object[%2d] = %p: %s "
 				   "%08x %08zx %08x %s %08x %08x %d",
 				   i, obj, get_pin_flag(obj_priv),