media: staging/intel-ipu3: make imgu use fixed running mode

Currently, the imgu running mode need user to set by v4l2 ctrl.
However, imgu only support 2 pipes and 2 operation modes - video and
still. This patch make the first imgu subdev running as video and second
one running as still, it will make the user understand easily, it can
also cover current camera use cases requirement. The running mode is set
during subdev registering, no race-condition after change, so it is safe
to change the mode data type to integer.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>