media: i2c: imx290: Factor out format retrieval to separate function

The driver duplicates the same pattern to access the try or active
format in multiple locations. Factor it out to a separate function.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>