drm/msm: enable vblank during atomic commits

This has roughly the same effect as drm_atomic_helper_wait_for_vblanks(),
basically just ensuring that vblank accounting is enabled so that we get
valid timestamp/seqn on pageflip events.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Tested-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>