drm/i915: Disable self-refresh for untiled fbs on i915gm

Apparently it doesn't work. X-tiled self-refresh works flawlessly
otoh. Apparently X still works correctly with linear framebuffers, so
might just be an issue with the initial modeset. It's unclear whether
this just borked wm setup from our side or a hw restriction, but just
disabling gets things going.

Note that this regression was only brought to light with

commit 3f2dc5ac
Author: Ville Syrjälä <ville.syrjala@linux.intel.com>
Date:   Fri Jan 10 14:06:47 2014 +0200

    drm/i915: Fix 915GM self-refresh enable/disable

before that self-refresh for i915GM didn't work at all.

Kudos to Ville for spotting a little bug in the original patch I've
attached to the bug.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76103


Tested-by: Krzysztof Mazur <krzysiek@podlesie.net>
Cc: Krzysztof Mazur <krzysiek@podlesie.net>
Cc: stable@vger.kernel.org
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[Jani: rebase on top of drm-next with primary plane support.]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>