media: v4l2-mem2mem: Fix missing v4l2_m2m_try_run call

Commit 34dbb848 ("media: mem2mem: Remove excessive try_run call")
removed a redundant call to v4l2_m2m_try_run but instead introduced
a bug. Consider the following case:

 1) Context A schedules, queues and runs job A.
 2) While the m2m device is running, context B schedules
    and queues job B. Job B cannot run, because it has to
    wait for job A.
 3) Job A completes, calls v4l2_m2m_job_finish, and tries
    to queue a job for context A, but since the context is
    empty it won't do anything.

In this scenario, queued job B will never run. Fix this by calling
v4l2_m2m_try_run from v4l2_m2m_try_schedule.

While here, add more documentation to these functions.

Fixes: 34dbb848 ("media: mem2mem: Remove excessive try_run call")

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
[hans.verkuil@cisco.com: split >80 cols line]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>