gpu: nvgpu: wait for running jobs to finish before shutdown

gpu: nvgpu: wait for running jobs to finish before shutdown

In gk20a_pm_shutdown(), we currently call __pm_runtime_disable()
which prevents h/w access to new requests made after shutdown() call
Also, once gk20a_pm_shutdown() completes, platform code will
just rail gate the GPU

But it is possible that some other thread is already accessing
h/w while shutdown() was triggered and this can result in hang

Hence, wait until all currently executing jobs are finished
before returning from gk20a_pm_shutdown()

Also, we need to wait for GPU's usage count to become 1 since
platform code will increase the usage count and then call
shutdown(). Hence usage count of 1 indicates that GPU is idle

Bug 200099940

Change-Id: I1f2457829e2737c07302d13f355353a30c3b4e67
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/734920
Reviewed-by: Bharat Nihalani <bnihalani@nvidia.com>
Tested-by: Bharat Nihalani <bnihalani@nvidia.com>