drm/i915: stop using dev->agp->base

For that to work we need to export the base address of the gtt
mmio window from intel-gtt. Also replace all other uses of
dev->agp by values we already have at hand.

Reviewed-by: Jani Nikula <jani.nikula@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.c b/drivers/gpu/drm/i915/intel_ringbuffer.c
index 89a5e7f..14025ab 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.c
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.c
@@ -968,6 +968,7 @@
 				  struct intel_ring_buffer *ring)
 {
 	struct drm_i915_gem_object *obj;
+	struct drm_i915_private *dev_priv = dev->dev_private;
 	int ret;
 
 	ring->dev = dev;
@@ -997,8 +998,9 @@
 	if (ret)
 		goto err_unref;
 
-	ring->virtual_start = ioremap_wc(dev->agp->base + obj->gtt_offset,
-					 ring->size);
+	ring->virtual_start =
+		ioremap_wc(dev_priv->mm.gtt->gma_bus_addr + obj->gtt_offset,
+			   ring->size);
 	if (ring->virtual_start == NULL) {
 		DRM_ERROR("Failed to map ringbuffer.\n");
 		ret = -EINVAL;