drm/msm/sde: Correct initialization value for frame count

For calculating fps, correct frame count initialization
from 1 to 0 to avoid counting one extra frame on every
periodicity window.

Change-Id: I3da5cc1f810da6b2f75b98772f116f5c1be433a9
Signed-off-by: Shubhashree Dhar <dhar@codeaurora.org>