media: ipu3-cio2: Fix PM runtime usage_count in driver unbind

Get the PM runtime usage_count and forbid PM runtime at driver unbind. The
opposite is being done in probe() already.

Fixes: commit c2a6a07a

 ("media: intel-ipu3: cio2: add new MIPI-CSI2 driver")
Cc: stable@vger.kernel.org # for >= 4.16
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>