media: atomisp: Use vb2_get_buffer() instead of directly access to buffers array

Use vb2_get_buffer() instead of direct access to the vb2_queue bufs array.
This allows us to change the type of the bufs in the future.
No need to check the result of vb2_get_buffer, vb2_ioctl_dqbuf() already
checked that it is valid.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
CC: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>