[media] media: check status of dmxdev->exit in poll functions of demux&dvr

when usb-type tuner is pulled out, user applications did not close device's FD,
and go on polling the device, we should return POLLERR directly.

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