V4L/DVB: cx18: Add missing serialization locks to cx18-dvb

Operations that manipulates the state of the encoder or global instance flags
and variables should be serialized.  This adds some serialization locks that
have been missing from cx18-dvb.  Since the DVB part of the CX23418 doesn't
affect the encoder, no one would likely have ever noticed a race in changing
card instance variables.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>