drm/i915: Disable semaphores by default on SNB

Semaphores still cause problems on some machines:

> From Udo Steinberg:
>
> With Linux-3.2-rc6 I'm frequently seeing GPU hangs when large amounts of
> text scroll in an xterm, such as when extracting a tar archive. Such as this
> one (note the timestamps):
>
>  I can reproduce it fairly easily with something
>  as simple as:
>
>	  while true; do dmesg; done

This patch turns them off on SNB while leaving them on for IVB.
Reported-by: NUdo Steinberg <udo@hypervisor.org>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Eugeni Dodonov <eugeni@dodonov.net>
Signed-off-by: NKeith Packard <keithp@keithp.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/gpu/drm/i915/i915_gem_execbuffer.c

@@ -756,9 +756,9 @@ intel_enable_semaphores(struct drm_device *dev)
 	if (i915_semaphores >= 0)
 		return i915_semaphores;
 
-	/* Enable semaphores on SNB when IO remapping is off */
+	/* Disable semaphores on SNB */
 	if (INTEL_INFO(dev)->gen == 6)
-		return !intel_iommu_enabled;
+		return 0;
 
 	return 1;
 }