drm/msm/mdp4: request vblank during modeset

This avoids a problem seen with weston (for example) where the display
gets stuck in "black screen" if starting weston first thing after boot.
Possibly mdp5 needs something similar.  The downstream android fbdev
driver always requests DMA_E (or DMA_P) when display is active, rather
than only enabling it on-demand as the drm driver does, which I believe
has the same end result.

Signed-off-by: Rob Clark <robdclark@gmail.com>