msm: mdss: fix spurious wait4pingpong timeouts

In some cases, the jiffies of the wait function can
jump between reads, leading to wrongly detected
ping pong timeouts. Prevent to fail in this scenario
by making sure that the time elapsed during the
wait is valid.

CRs-Fixed: 1048727
Change-Id: I3a1ecc89f379a90d9fdacf0baa9b6c8498bb93fb
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>