drivers: video: fix order of fields to allow use with FIFOs

The struct video_buffer are currently used with `struct k_fifo`, which
require them to preserve the first word of data for use by the kernel.
Move the first fields to make it possible to keep using video buffers in
FIFO safely.

Fixes #92526

Signed-off-by: Josuah Demangeon <me@josuah.net>