[media] stb6100: get rid of get_state()/set_state()

It is tricky to get rid of those ops here, as the stv0299 driver
wants to set frequency in separate from setting the bandwidth.

So, we use a small trick: we temporarely fill the cache with
0 for either frequency or bandwidth and add some logic at
set_params to only change the property(ies) that aren't zero.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>