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 5b680403 ("drm/rockchip: Enable
vblank without event").
Signed-off-by: NTomasz Figa <tfiga@chromium.org>