media: imx274: add helper to read multibyte registers

Currently 2-bytes and 3-bytes registers are read one byte at a time,
doing the needed shift & mask each time.

Replace all of this code by a unique helper function that calls
regmap_bulk_read(), which has two advantages:
 - reads all the bytes in a unique I2C transaction
 - simplifies code to read multibyte registers

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>