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

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 3f2dc5ac05714711fc14f2bf0ee5e42d5c08c581
Author: Ville Syrjälä <[email protected]>
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 <[email protected]>
Cc: Chris Wilson <[email protected]>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76103
Tested-by: Krzysztof Mazur <[email protected]>
Cc: Krzysztof Mazur <[email protected]>
Cc: [email protected]
Reviewed-by: Ville Syrjälä <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
[Jani: rebase on top of drm-next with primary plane support.]
Signed-off-by: Jani Nikula <[email protected]>