media: v4l2-subdev: fix some NULL vs IS_ERR() checks

The v4l2_subdev_alloc_state() function returns error pointers, it
doesn't return NULL.

Fixes: 0d346d2a ("media: v4l2-subdev: add subdev-wide state struct")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>