agp/intel: use correct GTT offset on VLV

VLV is a gen7 device, but we don't currently handle that in the switch.
So add it and write the PTEs correctly.
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/char/agp/intel-gtt.c

@@ -1253,6 +1253,7 @@ static int i9xx_setup(void)
 		switch (INTEL_GTT_GEN) {
 		case 5:
 		case 6:
+		case 7:
 			gtt_offset = MB(2);
 			break;
 		case 4: