drm/i915: Disable high-precision vblank timestamping for UMS

We only have sufficient information for accurate (sub-frame) timestamping
when the modesetting is under our control.

Reported-by: Chris Clayton <chris2553@googlemail.com>
Tested-by: Chris Clayton <chris2553@googlemail.com>
Reviewed-by: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>