drm/i915: Fix resume regression from 5d1d0cc

The irony of the patch to fix the resume regression on PineView causing
a further regression on Ironlake is not lost on me.

Reported-by: Jeff Chua <jeff.chua.linux@gmail.com>
Reported-by: Björn Schließmann <chronoss@gmx.de>
Tested-by: Björn Schließmann <chronoss@gmx.de>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=28802
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>