drm/i915: Wrap vma->pin_count accessors with small inline helpers

In the next few patches, the VMA pinning API is overhauled and to reduce
the churn we pull out the update to the accessors into a prep patch.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470324762-2545-14-git-send-email-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/i915_gem_fence.c b/drivers/gpu/drm/i915/i915_gem_fence.c
index dbaab9c..3b462da 100644
--- a/drivers/gpu/drm/i915/i915_gem_fence.c
+++ b/drivers/gpu/drm/i915/i915_gem_fence.c
@@ -431,7 +431,7 @@
 
 		WARN_ON(!ggtt_vma ||
 			dev_priv->fence_regs[obj->fence_reg].pin_count >
-			ggtt_vma->pin_count);
+			i915_vma_pin_count(ggtt_vma));
 		dev_priv->fence_regs[obj->fence_reg].pin_count++;
 		return true;
 	} else