gpu: nvgpu: make jobs_lock more fine grained

gpu: nvgpu: make jobs_lock more fine grained

While processing all the jobs in gk20a_channel_clean_up_jobs(),
We currently acquire jobs_lock, traverse the list,
clean up the jobs, and then release the lock

But in this case we might hold the lock for too long
blocking the submit path

Hence make jobs_lock more fine grained by restricting
it for list accesses only

Bug 200187553

Change-Id: If82af8ff386f7bc29061cfd57fdda7df62f11c17
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/1120412
(cherry picked from commit 2ac8c9729a5b7ca0b0bdc053e72d2b4658f8bed7)
Reviewed-on: http://git-master/r/1139543
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>