disp: 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: I53cea321bb4d3335cc58e08df2f530cd1a306297
Signed-off-by: Shubhashree Dhar <dhar@codeaurora.org>