gpu: nvgpu: gk20a: Use spin_lock for jobs_lock

gpu: nvgpu: gk20a: Use spin_lock for jobs_lock

This is done to boost performance of the GPU submit time, which
is critical for compute use-cases.

Bug 200215465

Change-Id: Ic4884ee4eac910b92b84a47fdc1b2e9f26b2f1f0
Signed-off-by: Bharat Nihalani <bnihalani@nvidia.com>
Reviewed-on: http://git-master/r/1199860
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>
Reviewed-on: http://git-master/r/1201496
GVS: Gerrit_Virtual_Submit