media: atomisp: Remove snr_mbus_fmt local var from atomisp_try_fmt()

snr_mbus_fmt is a local variable pointing to a substruct of another local
variable which really just makes the code harder to read / follow,
so drop it.

And likewise also drop the stream_info local variable.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>