drm/rockchip: Do not enable vblank without event

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: Tomasz Figa <tfiga@chromium.org>