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

This reverts commit e878742c.

Imgu should still keep the capability and flexibility to allow user to
run 2 video pipes, as the user may use the video pipe to capture still
frames with less system load and power than still pipe.

Suggested-by: Tomasz Figa <tfiga@chromium.org>
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>