drm/rockchip: Always signal event in next vblank after cfg_done

This patch makes the driver send the pending vblank event in next vblank
following the commit, relying on vblank signalling improvements done in
previous patches. This gives us vblank events that always represent the
real moment of changes hitting on the screen (which was the case only
for complete FB changes before) and lets us remove the manual window
update check.

Signed-off-by: Tomasz Figa <tfiga@chromium.org>