msm: display: dsi command mode vsync time generation

In each vsync cycle there will be two vsync interrupts
generated, need ignore the wrong one to provide the correct vsync
timestamp.

CRs-fixed: 452269
Change-Id: Ic13eea77d15eb46f4879f818d346425c63efe41a
Signed-off-by: Ken Zhang <kenz@codeaurora.org>