media: atomisp: fix try_fmt logic

[ Upstream commit c9e9094c ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>