drivers: soc: qcom: update rimps memlat while saving pmu counters

During hoplug cpu going down path pmu event handles will be freed.
If the hotplug path and idle path run in parallel there are chances
in idle path that the driver will try to read the PMU counters with
freed pointer resulting in a crash.

Add NULL pointer check before performing PMU reads.

Change-Id: I9b8a4a8dfa63e70e879c3530a878a9c96062872f
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>