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/intel_overlay.c b/drivers/gpu/drm/i915/intel_overlay.c
index 60595fc..6d524a1 100644
--- a/drivers/gpu/drm/i915/intel_overlay.c
+++ b/drivers/gpu/drm/i915/intel_overlay.c
@@ -724,7 +724,7 @@
 	int ret, tmp_width;
 	struct overlay_registers *regs;
 	bool scale_changed = false;
-	struct drm_i915_gem_object *bo_priv = new_bo->driver_private;
+	struct drm_i915_gem_object *bo_priv = to_intel_bo(new_bo);
 	struct drm_device *dev = overlay->dev;
 
 	BUG_ON(!mutex_is_locked(&dev->struct_mutex));
@@ -809,7 +809,7 @@
 	intel_overlay_continue(overlay, scale_changed);
 
 	overlay->old_vid_bo = overlay->vid_bo;
-	overlay->vid_bo = new_bo->driver_private;
+	overlay->vid_bo = to_intel_bo(new_bo);
 
 	return 0;
 
@@ -1344,7 +1344,7 @@
 	reg_bo = drm_gem_object_alloc(dev, PAGE_SIZE);
 	if (!reg_bo)
 		goto out_free;
-	overlay->reg_bo = reg_bo->driver_private;
+	overlay->reg_bo = to_intel_bo(reg_bo);
 
 	if (OVERLAY_NONPHYSICAL(dev)) {
 		ret = i915_gem_object_pin(reg_bo, PAGE_SIZE);