drm: make sure vblank interrupts are disabled at DPMS time

When we call drm_vblank_off() at DPMS off time (to wake any clients so
they don't hang) we need to make sure interrupts are actually disabled.
If drm_vblank_off() gets called before the vblank usage timer expires,
it'll prevent the timer from disabling interrupts since it also clears
the vblank_enabled flag for the pipe.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>