msm: mdss: fix use of uninitialized local variable

This change fixes unitilized local variable being used and
checks for the valid format in concurrent writeback case before
using the format parameters.

CRs-Fixed: 1110015
Change-Id: I61a3e20af1d0a4db7a7db454ae35b810639744ae
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>