media: atomisp: Fix VIDIOC_REQBUFS failing when VIDIOC_S_FMT has not been called yet

camorama calls VIDIOC_REQBUFS(V4L2_MEMORY_MMAP) to test if MMAP support
works (this was added specifically because of the previously broken
MMAP support in atomisp).

Currently this fails because atomisp_get_css_frame_info() fails in this
case. Although it is weird to call VIDIOC_REQBUFS before VIDIOC_S_FMT,
it is allowed to do this.

Fix this not working by doing a S_FMT to V4L2_PIX_FMT_YUV420 + the highest
supported resolution.

Note this will cause camorama to use mmap mode, which means it will also
use libv4l2 to do format conversion. libv4l2 will pick V4L2_PIX_FMT_RGB565
as input format and this will lead to a garbled video display. This is
a libv4lconvert bug, the RGB565 -> RGB24 path in libv4lconvert assumes
that stride == width which is not true on the atomisp.

I've already send out a libv4lconvert fix for this. Also this can be worked
around by passing --dont-use-libv4l2 as argument to camorama.

Link: https://git.linuxtv.org/v4l-utils.git/commit/?id=aecfcfccfc2f78d7531456ffa5465666c6bc641e



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