drm/radeon: check incoming cliprects pointer

commit fefaedcf upstream.

The "boxes" parameter points into userspace memory. It should be verified
like any other operation against user memory.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>