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>

include/drm/intel-gtt.h

@@ -19,6 +19,8 @@ const struct intel_gtt {
 	dma_addr_t scratch_page_dma;
 	/* for ppgtt PDE access */
 	u32 __iomem *gtt;
+	/* needed for ioremap in drm/i915 */
+	phys_addr_t gma_bus_addr;
 } *intel_gtt_get(void);
 
 void intel_gtt_chipset_flush(void);