[media] m88ds3103: use regmap for I2C register access

Use regmap for I2C register access.
Remove own I2C repeated mutex as it should not be needed. I2C adapter
lock is already taken when I2C mux adapter is called, no need for
double locking.

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