drm/i915/lvds: Only act on lid notify when the device is on

If we're using vga switcheroo, the device may be turned off
and poking it can return random state. This provokes an OOPS fixed
separately by 8ff887c847 (drm/i915/dp: Be paranoid in case we disable a
DP before it is attached). Trying to use and respond to events on a
device that has been turned off by the user is in principle a silly thing
to do.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org
Signed-off-by: Keith Packard <keithp@keithp.com>