check compositor screen iface existence before deref in dpms
check compositor screen iface existence before deref in dpms

fixes crash in compositors lacking screen ifaces

rvandegrift/e,tizenorg/platform.upstream.enlightenment,tasn/enlightenment,tasn/enlightenment,tizenorg/platform.upstream.enlightenment,rvandegrift/e,rvandegrift/e,tizenorg/platform.upstream.enlightenment,tasn/enlightenment