media: v4l2-mem2mem: Document that v4l2_m2m_get_vq() never returns NULL

media: v4l2-mem2mem: Document that v4l2_m2m_get_vq() never returns NULL

The v4l2_m2m_get_vq() never returns a NULL pointer, as the internal
get_queue_ctx() helper always returns a non-NULL pointer. Many drivers
check the return value against NULL, due to a combination of old code
and cargo-cult programming. Even v4l2-mem2mem.c contains unneeded NULL
checks.

Clarify the API by documenting explicitly that a NULL check is not
needed, and simplify the code by removing the unneeded NULL checks from
v4l2-mem2mem.c.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Stefan Klug <stefan.klug@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>