gpu: nvgpu: return from worker if gpu is not up

gpu: nvgpu: return from worker if gpu is not up

During GPU shutdown path, it is possible that we
shut down the GPU while worker thread is still
running gk20a_channel_update()

Hence before accessing gp_put/get, check if GPU
is up or not

Bug 200166139

Change-Id: Iba3ec173041a84527c4700a93f20564a842cfb01
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/935193
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>