nf: IDLETIMER: Fix possible use before initialization in idletimer_resume

idletimer_resume() assumes that the PM_SUSPEND_PREPARE notifier is sent
before PM_POST_SUSPEND so that timer->last_suspend_time is initialized.
However, it is possible for PM_POST_SUSPEND to be sent first if there is an
error returned from another driver's PM_SUSPEND_PREPARE notifier.

Add a flag indicating whether the current value of timer->last_suspend is
valid.

Detected with CONFIG_SLUB_DEBUG & CONFIG_DEBUG_SPINLOCK in arm64. The
timestamp lock is held for more than a minute while
set_normalized_timespec() proceses the poisoned timer->last_suspend_time
argument.

CRs-Fixed: 2333588
Change-Id: I95328b0ac85dba819ff9cef751c3d07300c232f1
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>