media: ipu3-imgu: Set valid initial format

The initial format did not have a valid size.

Fixes v4l2-compliance:

fail: v4l2-test-formats.cpp(723): Video Output Multiplanar:
				  TRY_FMT(G_FMT) != G_FMT
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>