drm/i915: don't enable vblanks on disabled pipes

commit 71e0ffa5 upstream.

In some cases userland may be confused and try to wait on vblank events from
pipes that aren't actually enabled.  We shouldn't allow this, so return
-EINVAL if the pipe isn't on.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Dave Airlie <airlied@linux.ie>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>