drm/i915: vlv: fix VGA hotplug after modeset

Since

commit 912d812e
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Oct 11 20:08:23 2012 +0200

    drm/i915/crt: don't set HOTPLUG bits on !PCH

on VLV we don't detect any VGA unplug event after a modeset, since there we
reset the ADPA hotplug bits. Fix it by preserving the hotplug bits on VLV as
well.

Signed-off-by: Imre Deak <imre.deak@intel.com>
[danvet: For consistency use gen >= 5 like in Chris' exact same fix
in intel_crt_reset.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>