drm/i915: Use a separate slab for vmas

vma are more frequently allocated than objects and so should equally
benefit from having a dedicated slab.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index 855eaac..b675075 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -1559,6 +1559,7 @@
 struct drm_i915_private {
 	struct drm_device *dev;
 	struct kmem_cache *objects;
+	struct kmem_cache *vmas;
 	struct kmem_cache *requests;
 
 	const struct intel_device_info info;