xf86atomic: require CAS support in libatomic_ops

Since AO_compare_and_swap_full() is used by libdrm, AO_REQUIRE_CAS
must be defined before including <atomic_ops.h> so that we are sure
that CAS support will be provided. This is necessary to make sure that
the AO_compare_and_swap_full() function will be provided on all
architectures, including the ones that don't have built-in CAS support
such as SPARCv8.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Acked-by: Eric Engestrom <eric.engestrom@intel.com>