drm/i915/bdw: Restore PPAT on thaw

Apparently it is wiped out from under us, and we get some really fun
caching artifacts upon resume (it seems to be WB for all types by
default).

Reported-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Tested-by: James Ausmus <james.ausmus@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76113


Tested-by: Timo Aaltonen <timo.aaltonen@canonical.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>