gpu: nvgpu: fix possible PMU isr race

gpu: nvgpu: fix possible PMU isr race

Possible race description :
- while PMU is booting, it sends messages to kernel which we process
  in gk20a_pmu_isr()
- but when messages are processed it is possible that we are on the way
  to rail gate the GPU and we have already called pmu_destroy()
- this could lead to hangs if while processing messages, GR is
  already off

To fix this, introduce another mutex isr_enable_lock and a flag to
turn on/off ISRs
- when we enable PMU, get the lock and set the flag
- in pmu_destroy(), get the lock and remove the flag
- in pmu_isr(), take the lock, check if flag is set or not. If flag
  is not set return, otherwise proceed with the messages

Bug 200014542
Bug 200014887

Change-Id: I0204d8a00e4563859eebc807d4ac7d26161316ea
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/428371
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Vijayakumar Subbu <vsubbu@nvidia.com>
Reviewed-by: Terje Bergstrom <tbergstrom@nvidia.com>