drm/i915: fix OOPS in lid_notify

This goes back to

commit c1c7af60
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Thu Sep 10 15:28:03 2009 -0700

    drm/i915: force mode set at lid open time

It was used to fix an issue on a i915GM based Thinkpad X41, which
somehow clobbered the modeset state at lid close time. Since then
massive amounts of things changed: Tons of fixes to the modeset
sequence, OpRegion support, better integration with the acpi code.
Especially OpRegion /should/ allow us to control the display hw
cooperatively with the firmware, without the firmware clobbering the
hw state behind our backs.

So it's dubious whether we still need this.

The second issue is that it's unclear who's responsibility it actually
is to restore the mode - Chris Wilson suggests to just emit a hotplug
event and let userspace figure things out.

The real reason I've stumbled over this is that the new modeset code
breaks drm_helper_resume_force_mode - it OOPSes derefing a NULL vfunc
pointer. The reason this wasn't caught in testing earlier is that in

commit c9354c85
Author: Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon Nov 2 09:29:55 2009 -0800

    i915: fix intel graphics suspend breakage due to resume/lid event
    confusion

logic was added to _not_ restore the modeset state after a resume. And
since most machines are configured to auto-suspend on lid-close, this
neatly papered over the issue.

Summarizing, this shouldn't be required on any platform supporting
OpRegion. And none of the really old machines I have here seem to
require it either. Hence I'm inclined to just rip it out.

But in case that there are really firmwares out there that clobber the
hw state, replace it with a call to intel_modset_check_state. This
will ensure that we catch any issues as soon as they happen.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>