drm/i915: Disable GPU semaphores on SandyBridge mobile

Hopefully, this is a temporary measure whilst the root cause is
understood. At the moment, we experience a hard hang whilst looping
urbanterror that has been identified as a result of the use of
semaphores, but so far only on SNB mobile.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=32752


Tested-by:  <mengmeng.meng@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>