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.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 8cd8aab..d4e33ac 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -1999,6 +1999,7 @@
 	else
 		iounmap(ringbuf->virtual_start);
 	ringbuf->virtual_start = NULL;
+	ringbuf->vma = NULL;
 	i915_gem_object_ggtt_unpin(ringbuf->obj);
 }
 
@@ -2065,6 +2066,8 @@
 		}
 	}
 
+	ringbuf->vma = i915_gem_obj_to_ggtt(obj);
+
 	return 0;
 }