drm/i915: Disable waitboosting for fence_wait()

We want to restrict waitboosting to known process contexts, where we can
track which clients are receiving waitboosts and prevent excessive power
wasting. For fence_wait() we do not have any client tracking and so that
leaves it open to abuse.

v2: Hide the IS_ERR_OR_NULL testing for special clients

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469002875-2335-5-git-send-email-chris@chris-wilson.co.uk