drm/radeon: async event synchronization for drmWaitVblank

Bring radeon up to speed with the async event synchronization for
drmWaitVblank. See c9a9c5e0

 for
more information. Without this patch event never get delivered
to userspace client.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>