media: atomisp: avoid a copy of v4l2_mbus_framefmt at stack

There's no reason to copy isp_sink_fmt, as the driver
uses it for read-only purposes.

Linux stack is a precious resource. Let's avoid wasting it.

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