[media] dmxdev: don't use before checking file->private_data

As reported by smatch:
	drivers/media/dvb-core/dmxdev.c:1091 dvb_demux_poll() warn: variable dereferenced before check 'dmxdevfilter' (see line 1088)

This was introduced by changeset d102cac8.

We need to test before using it.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>