media: atomisp: Make pointer to PMIC client global

When we enumerate second device when PMIC has been successfully detected
to AXP, we got into troubles dereferencing NULL pointer. It seems
we have to detect PMIC only once because pmic_id is a global variable
and code doesn't expect the change of it: Two PMICs on one platform?
It's impossible.

Crash excerpt:
[   34.335237] BUG: kernel NULL pointer dereference, address: 0000000000000002
...
[   35.652036] RIP: 0010:gmin_subdev_add.cold+0x32f/0x33e [atomisp_gmin_platform]

So, as a quick fix make power a global variable. In next patches we move
PMIC detection to be more independent from subdevices.

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