msm_fb: display: skip one vsync event after enabling mdp and vsync clk

Both mdp and mdp_vsync clks are disabled for command mode panel when
no new frames are being played. When they are re-enabled if new
requests come in, the first vsync event which is signaled by read
pointer interrupt arrives in a random time due to the sync between mdp
and panel, so skip it to maintain correct vsync timing.

Change-Id: I683b765e4a16d3d152b9270f5982234b94921f1e
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>