Originally we needed to enable vblank for any atomic commit to kick the PSR machine, but that was changed and we no longer need to do so from a vblank interrupt. Let's return to original behavior of enabling vblank only if it is really necessary. This essentially reverts commit 5b6804034ae9 ("drm/rockchip: Enable vblank without event"). Signed-off-by: Tomasz Figa <tfiga@chromium.org>