drm/agp: move AGP cleanup paths to drm_agpsupport.c

Introduce two new helpers, drm_agp_clear() and drm_agp_destroy() which
clear all AGP mappings and destroy the AGP head. This allows to reduce the
AGP code in core DRM and move it all to drm_agpsupport.c.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/drm_stub.c b/drivers/gpu/drm/drm_stub.c
index 327ca19..d663f7d 100644
--- a/drivers/gpu/drm/drm_stub.c
+++ b/drivers/gpu/drm/drm_stub.c
@@ -451,16 +451,11 @@
 
 	drm_lastclose(dev);
 
-	if (drm_core_has_MTRR(dev) && drm_core_has_AGP(dev) && dev->agp)
-		arch_phys_wc_del(dev->agp->agp_mtrr);
-
 	if (dev->driver->unload)
 		dev->driver->unload(dev);
 
-	if (drm_core_has_AGP(dev) && dev->agp) {
-		kfree(dev->agp);
-		dev->agp = NULL;
-	}
+	if (dev->driver->bus->agp_destroy)
+		dev->driver->bus->agp_destroy(dev);
 
 	drm_vblank_cleanup(dev);