media: staging: atomisp: don't access a NULL var

Get rid of those warnings:
	drivers/staging/media/atomisp/platform/intel-mid/atomisp_gmin_platform.c:446 gmin_v1p2_ctrl() error: we previously assumed 'gs' could be null (see line 444)
	drivers/staging/media/atomisp/platform/intel-mid/atomisp_gmin_platform.c:480 gmin_v1p8_ctrl() error: we previously assumed 'gs' could be null (see line 478)
	drivers/staging/media/atomisp/platform/intel-mid/atomisp_gmin_platform.c:516 gmin_v2p8_ctrl() error: we previously assumed 'gs' could be null (see line 514)

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>