i915: ensure that VGA plane is disabled

commit 0fde901f1ddd2ce0e380a6444f1fb7ca555859e9 upstream.

Some broken systems (like HP nc6120) in some cases, usually after LID
close/open, enable VGA plane, making display unusable (black screen on LVDS,
some strange mode on VGA output). We used to disable VGA plane only once at
startup. Now we also check, if VGA plane is still disabled while changing
mode, and fix that if something changed it.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=57434
Signed-off-by: Krzysztof Mazur <krzysiek@podlesie.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[bwh: Backported to 3.2: intel_modeset_setup_hw_state() does not
 exist, so call i915_redisable_vga() directly from intel_lid_notify()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Qiang Huang <h.huangqiang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>