msm: mdss: clear vsync interrupt bit before wait for completion

When kickoff happens, MDP needs to ensure that a vsync has happend after
flush. To ensure this, MDP driver needs to clear any pending vsync
interrupts after writing to flush to ensure that vsync has happened
and the shadowed registers have been swapped (at vsync boundary). This
addressed possible concurrency issue where vsync interrupt happens just
before flush but irq handler does not process it until after, leading to
iommu page fault.

CRs-Fixed: 465340
Change-Id: I0f8412386c76fc6409e95f316f54999f42b09be5
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>