msm: mdss: double check WB done status

There are cases when WB done might be set in MDP HW but the irq
trigger is delayed. Add an additional check in case of WB timeout,
to check if the WB done is set.

Change-Id: I1cdb2c85157d4c66718deae8e49778b9f3962f5d
Signed-off-by: Krishna Chaitanya Devarakonda <kdevarak@codeaurora.org>