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/i915_drv.h b/drivers/gpu/drm/i915/i915_drv.h
index ccabadd..ae4129b 100644
--- a/drivers/gpu/drm/i915/i915_drv.h
+++ b/drivers/gpu/drm/i915/i915_drv.h
@@ -651,6 +651,7 @@
 		unsigned long gtt_end;
 
 		struct io_mapping *gtt_mapping;
+		phys_addr_t gtt_base_addr;
 		int gtt_mtrr;
 
 		/** PPGTT used for aliasing the PPGTT with the GTT */