Don't enable vblank if the pipe is disabled.
Don't enable vblank if the pipe is disabled.

Based on a patch from the intel driver maintainers (Jesse Barnes,
specifically)

Tested by several people on various hardware.

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars