drm/i915: enable semaphores on gen6 if dmar is not active

Inspired by the recent ppgtt regression report, where switching of
dmar only for the gpu seems to fix things completely, I've looked
again at the semaphores+vt-d situation.

Contrary to my earlier testing a few months back my system is now
stable with dmar disabled for the igd, and not only when disabling
dmar completely.

So I'm rather hopeful that all our recent fixes for snb have changed
things for code and it's time to try enabling semaphores again. We've
also had issues with enabling semaphores which are not vt-d related,
but I guess these are all fixed by the autoreport-disabling and lazy
request fix. And there's only one way to find out whether there are
still other issues ...

When I've tried to apply this patch I've noticed that semaphores on
gen6 have already silently been enabled in

commit 2911a35b
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Thu Apr 5 14:47:36 2012 -0700

    drm/i915: use semaphores for the display plane

Fix this up by only checking whether dmar is enabled on the gfx (not
on the entire system).

Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>