PM / devfreq: gpubw_mon: Add null check for governor private data

During SUSPEND event, check the pointer for governor
private data is not NULL before updating bus parameters
to zero.

Change-Id: I1a37173e8ae7ad4bcd5f8497c5956302e647c862
Signed-off-by: Archana Sriram <apsrir@codeaurora.org>