drm: msm: sde: improve display interface timeout mechanism

When enabling splash in LK, after kernel boots up, sometimes
interface timeout issue may happen. This is caused by kernel's
defect when decreasing pending frame kickoff count in vblank isr.
As LK and kernel may concurrently flush pipes, so flush_bit may
be set by LK, while kernel still treats a timeout issue if it's
not 0. This change is to fix the defect.

Change-Id: I928e444074617adf2dc86134dd3a96701c8f6aca
Signed-off-by: Guchun Chen <guchunc@codeaurora.org>