media: staging/intel-ipu3: set the main output as mandatory

From firmware perspective, the main output for each pipe is mandatory
and must be enabled. This patch set the link between imgu subdev and
main output as IMMUTABLE by default.

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>