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>

drivers/gpu/drm/i915/i915_drv.c

@@ -752,6 +752,9 @@ static int __init i915_init(void)
 		driver.driver_features &= ~DRIVER_MODESET;
 #endif
 
+	if (!(driver.driver_features & DRIVER_MODESET))
+		driver.get_vblank_timestamp = NULL;
+
 	return drm_init(&driver);
 }