drm/radeon: fix build on 64-bit with some compilers.

drivers/gpu/drm/radeon/radeon_test.c:45: undefined reference to `__udivdi3'

Reported-by: Mr. James W. Laferriere <babydr@baby-dragons.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_test.c

@@ -42,8 +42,8 @@ void radeon_test_moves(struct radeon_device *rdev)
 	/* Number of tests =
 	 * (Total GTT - IB pool - writeback page - ring buffer) / test size
 	 */
-	n = (rdev->mc.gtt_size - RADEON_IB_POOL_SIZE*64*1024 - RADEON_GPU_PAGE_SIZE -
-	     rdev->cp.ring_size) / size;
+	n = ((u32)(rdev->mc.gtt_size - RADEON_IB_POOL_SIZE*64*1024 - RADEON_GPU_PAGE_SIZE -
+	     rdev->cp.ring_size)) / size;
 
 	gtt_obj = kzalloc(n * sizeof(*gtt_obj), GFP_KERNEL);
 	if (!gtt_obj) {