V4L/DVB (13975): [STV090x] Added internal structure with shared settings and data.

As the STV0900 features two demodulation paths in one chip there is
some information used by both instances of the driver when used in
dual mode. This information is now shared in an internal structure
referenced by I2C adapter and address.

Do initialisation of the demodulator only once when used in dual mode.
Moved global mutex demod_lock to internal structure.
Moved dev_ver and mclk to internal structure.
Removed unused tuner_refclk from stv090x_state.

Signed-off-by: Andreas Regel <andreas.regel@gmx.de>
Signed-off-by: Manu Abraham <manu@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>