media: video-i2c: use i2c regmap

Use regmap for i2c register access.  This simplifies register accesses and
chooses suitable access commands based on the functionality that the
adapter supports.

Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>