drm/msm: convert kms->complete_commit() to crtc_mask

Prep work for async commits, in which case this will be called after we
no longer have the atomic state object.

This drops some wait_for_vblanks(), but those should be unnecessary, as
we call this after waiting for flush to complete.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Sean Paul <sean@poorly.run>