gpu: nvgpu: Fix PMU destroy sequence

A call to exit the PMU state machine/kthread must
be prioritized over any other state change.
It was possible to set the state as PMU_STATE_EXIT,
signal the kthread and overwrite the state before
the kthread has had the chance to exit its loop.
This may lead to a "lost" signal, resulting in
indefinite wait during the destroy sequence.

Faulting sequence:
1. pmu_state = PMU_STATE_EXIT in nvgpu_pmu_destroy()
2. cond_signal()
3. pmu_state = PMU_STATE_LOADING_PG_BUF
4. PMU kthread wakes up
5. PMU kthread processes PMU_STATE_LOADING_PG_BUF
6. PMU kthread sleeps
7. nvgpu_pmu_destroy() waits indefinitely

This patch adds a sticky flag to indicate PMU_STATE_EXIT,
irrespective of any subsequent changes to pmu_state.

The PMU PG init kthread may wait on a call to
NVGPU_COND_WAIT_INTERRUPTIBLE, which requires a
corresponding call to nvgpu_cond_signal_interruptible()
as the core kernel code requires this task mask to
wake-up an interruptible task.

Bug 2658750
Bug 200532122

Change-Id: I61beae80673486f83bf60c703a8af88b066a1c36
Signed-off-by: Abhiroop Kaginalkar <akaginalkar@nvidia.com>
Reviewed-on: https://git-master.nvidia.com/r/2177112
(cherry picked from commit afa49fb073a324c49a820e142aaaf80e4656dcc6)
Reviewed-on: https://git-master.nvidia.com/r/2190733
Tested-by: Divya Singhatwaria <dsinghatwari@nvidia.com>
Reviewed-by: Debarshi Dutta <ddutta@nvidia.com>
Reviewed-by: Alex Waterman <alexw@nvidia.com>
GVS: Gerrit_Virtual_Submit
Reviewed-by: Bibek Basu <bbasu@nvidia.com>
Reviewed-by: mobile promotions <svcmobile_promotions@nvidia.com>
Tested-by: mobile promotions <svcmobile_promotions@nvidia.com>

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