media: atomisp: Remove atomisp_source_pad_to_stream_id()

atomisp_source_pad_to_stream_id() returns ATOMISP_INPUT_STREAM_GENERAL
unconditionally now. Drop it and directly use ATOMISP_INPUT_STREAM_GENERAL
in its callers.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>