drm/radeon/kms: fix vram base calculation on rs780/rs880

Avoid overflowing a 32 bit value.
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/r600.c

@@ -1195,8 +1195,10 @@ void r600_vram_gtt_location(struct radeon_device *rdev, struct radeon_mc *mc)
 				mc->vram_end, mc->real_vram_size >> 20);
 	} else {
 		u64 base = 0;
-		if (rdev->flags & RADEON_IS_IGP)
-			base = (RREG32(MC_VM_FB_LOCATION) & 0xFFFF) << 24;
+		if (rdev->flags & RADEON_IS_IGP) {
+			base = RREG32(MC_VM_FB_LOCATION) & 0xFFFF;
+			base <<= 24;
+		}
 		radeon_vram_location(rdev, &rdev->mc, base);
 		rdev->mc.gtt_base_align = 0;
 		radeon_gtt_location(rdev, mc);