drm/radeon: fix underflow in r600_cp_dispatch_texture()

The "if (pass_size > buf->total)" can underflow so I have changed the
type of size and pass_size to unsigned to avoid this problem.
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/r600_cp.c

@@ -2483,7 +2483,7 @@ int r600_cp_dispatch_texture(struct drm_device *dev,
 	struct drm_buf *buf;
 	u32 *buffer;
 	const u8 __user *data;
-	int size, pass_size;
+	unsigned int size, pass_size;
 	u64 src_offset, dst_offset;
 
 	if (!radeon_check_offset(dev_priv, tex->offset)) {