media: ov8856: apply digital gain by setting global gain control register

MWB gain register are used to set gain for each mwb channel mannually.
However, it will involve some artifacts if gain cannot be applied to
each channel synchronously. Enable global gain control to set digital
global gain instead of setting AWB gain separately to fix this issue.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>