atomisp: don't let it go past pipes array

In practice, IA_CSS_PIPE_ID_NUM should never be used when
calling atomisp_q_video_buffers_to_css(), as the driver should
discover the right pipe before calling it.

Yet, if some pipe parsing issue happens, it could end using
it.

So, add a WARN_ON() to prevent such case.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>