Revert "drm/i915: enable rc6 on ilk again"

Revert "drm/i915: enable rc6 on ilk again"

Even with the cumulative set of ilk w/a, rc6 is demonstrably still
failing and causing GPU hangs as found by Peter Wu. So we need to disable
it again until it is stable.

This reverts

commit 4812b72306426598b1987441b399c84efe59cec1
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Aug 8 23:35:40 2012 +0200

    drm/i915: enable rc6 on ilk again

and the follow-on

commit 7ccb531d5fd7651c33fc56b4d40bddfd56ced24f
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Sun Aug 26 20:33:18 2012 +0200

    drm/i915: disable rc6 on ilk when vt-d is enabled

Note: The situation around the gen4/5 gpu hangs that cropped up in 3.7
is rather strange. Most useful bisects have lead to

commit 5db5dccce10763373a9444dcc6896ce83ffcd237
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Mon Aug 20 11:40:46 2012 +0200

    drm/i915: Track unbound pages

or even later commits that affect the gem bo recycling, which all is
way past the point where we re-enabled rc6. But somehow
reverting/disabling those commits doesn't help, but disabling rc6 at
least helps for many hangs on ilk. Obviously it doesn't change
anything at all on gen4, and there are still strange issues left on
gen5 (which we unfortunately can't readily reproduce).

Also, the error_state signature of the hangs which can be fixed with
this patch look remarkably different to those which seem to be
unaffected by the rc6 settings: The rc6 hangs are in the ring,
somewhere in the MI_FLUSH/PIPE_CONTROL sequence to make ilk coherent,
wheras all the other hangs tend to be at a random point in the middle
of the user batch. So it could also be that we have different issues.

Until we grow more clue, this at least helps some users.

Reported-by: Peter Wu <lekensteyn@gmail.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=55984
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Added note with some more details about the gen4/5 3.7
gpu hang regression.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>