drm/rockchip: Enable vblank without event

vblank should be enabled regardless of whether an event
is expected back. This is especially important for a cursor
plane.

Reviewed-by: Yakir Yang <ykk@rock-chips.com>
Tested-by: Yakir Yang <ykk@rock-chip.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>