drm/radeon: Don't dereference possibly-NULL pointer.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/radeon/radeon_object.c

@@ -241,7 +241,8 @@ int radeon_bo_pin_restricted(struct radeon_bo *bo, u32 domain, u64 max_offset,
 				domain_start = bo->rdev->mc.vram_start;
 			else
 				domain_start = bo->rdev->mc.gtt_start;
-			WARN_ON_ONCE((*gpu_addr - domain_start) > max_offset);
+			WARN_ON_ONCE(max_offset <
+				     (radeon_bo_gpu_offset(bo) - domain_start));
 		}
 
 		return 0;