media: i2c: imx290: Group functions in sections

Move functions around to group them in logical sections, in order to
improve readability. As a result, the IMX290_NUM_SUPPLIES macro has to
be changed. No other code change is included, only moves.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>