media: v4l2-core: move i2c helpers out of v4l2-common.c

Separate the i2c helpers to v4l2-i2c.c, in order to get rid
of the ifdefery. No functional changes intended, this is
just a cosmetic change to organize the code better.

Given I2C is a tristate symbol, a hidden boolean symbol
is introduced, to make the conditional build easier.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>