drm/r100/kms: Emit cache flush to the end of command buffer. (v2)

Cache flush is required in case CPU is accessing rendered data.

This fixes glean/readPixSanity test case and random rendering
errors in sauerbraten and warzone2100.

v2 Fix comment ordering in r100_fence_ring_emit and remove extra
   defines added in first version.

Signed-off-by: Pauli Nieminen <suokkos@gmail.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>