media: ipu3-imgu: imgu_fmt: Handle properly try

For a try_fmt call, the node noes not need to be enabled.

Fixes v4l2-compliance

fail: v4l2-test-formats.cpp(717): Video Output Multiplanar is valid, but
				  no TRY_FMT was implemented
test VIDIOC_TRY_FMT: FAIL

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>