msm: isp: initialize pd stats buffer index

In dual vfe usecase pd_stats buffer index is valid
for one vfe only, for other vfe it should be set as
invalid value (0xF)

Change-Id: Idf7e11fa89fe447dec578dfac2906d2ce8f610c2
Signed-off-by: Srikanth Uyyala <suyyala@codeaurora.org>