drm/i915: Cache ringbuffer GTT VMA

Purpose is to avoid calling i915_gem_obj_ggtt_offset from the
interrupt context without the big lock held.

v2: Renamed gtt_start to gtt_offset. (Daniel Vetter)
v3: Cache the VMA instead of address. (Chris Wilson)

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1452870629-13830-2-git-send-email-tvrtko.ursulin@linux.intel.com
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 85ce227..ede5795 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -99,6 +99,7 @@
 struct intel_ringbuffer {
 	struct drm_i915_gem_object *obj;
 	void __iomem *virtual_start;
+	struct i915_vma *vma;
 
 	struct intel_engine_cs *ring;
 	struct list_head link;