gpu: nvgpu: force close all channels on shutdown

gpu: nvgpu: force close all channels on shutdown

In gk20a_pm_shutdown(), we currently wait for 2s
for all channels to finish their work

Instead of waiting, force close all channels
(disable and preempt) during shutdown

Also, if GPU is already railgated during shutdown,
then we can just disable runtime_pm and return
without doing anything

Bug 200166139

Change-Id: I0012f1b3c0f4f676958d083f8c60a001f7015fb0
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/931706
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>