msm:vidc:Reset average decode time after setting DPB buffers

In case of VGA/720p clips with higher frame rates, when
average decode time taken by the core is found to be
greater than 33ms, the clocks are bumped to the next level.
But the calculation of average decode time was erroneous,
due to which clocks started bumping up for other clips also.
This change resets the average decode time after setting
the DPB buffers and thus it corrects the calculation for
average decode time.

Change-Id: I3d7ab5a63e202a70a8e05f42dd4f989ce92b4bac
CRs-fixed: 409475
Signed-off-by: Deepak Verma <dverma@codeaurora.org>