[media] a8293: improve LNB register programming logic

On power-on LNB power supply voltage is disabled, due to that no
need to disable it during probe. Tone is supply is hard-coded as
external tone coming from the demodulator. Program both voltage
and tone on set_voltage(). Use register cache to prevent unneeded
programming.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>