media: atomisp: reorganize the code under gmin_subdev_add()

The gmin_subdev_add() currently doesn't use ACPI device
power management. In order to prepare for adding support
for it, let's shift some things, placing the PM-related
stuff at the end of the probing logic.

Let's also store the current gs on a temporary var, in
order to simplify the source code.

Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>