media: atomisp_gmin_platform: Unexport and split camera_sensor_csi()

The camera_sensor_csi() is not used outside the module, hence make it
static. While at it, split it to _alloc() and _free() to clearly show
the idea behind the last parameter @flag that is passed to
gmin_csi_cfg().

Link: https://lore.kernel.org/r/20220730162027.1011-1-andriy.shevchenko@linux.intel.com

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Tested-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>