gpu: nvgpu: return error from mutex_acquire() if pmu not initialized

gpu: nvgpu: return error from mutex_acquire() if pmu not initialized

In pmu_mutex_acquire(), we return zero (success) if
pmu->initialized is not set

Since mutex_acquire() was successful, we then call
pmu_mutex_release()

If now pmu->initialized is set in some other thread
then we proceed to validate the mutex owner and
end up causing below warning :

pmu_mutex_release: requester 0x00000000 NOT match owner 0x00000008

Hence to fix this return error from mutex_acquire()
and mutex_release() if pmu->initialized is not yet set
and in that case we proceed to call elpg enable/disable

Bug 1533644

Change-Id: Ifbb9e6a8e13f6478a13e3f9d98ced11792cc881f
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/439333
GVS: Gerrit_Virtual_Submit
Reviewed-by: Naveen Kumar S <nkumars@nvidia.com>
Tested-by: Naveen Kumar S <nkumars@nvidia.com>
Reviewed-by: Arto Merilainen <amerilainen@nvidia.com>