media: atomisp: fix try_fmt logic

The internal try_fmt logic is not meant to provide everything
that the V4L2 API should provide. Also, it doesn't decrement
the pads that are used only internally by the driver, but aren't
part of the device's output.

Fix it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>