drm/i915: Disable semaphore inter-engine sync without timeslicing

Since the removal of the no-semaphore boosting, we rely on timeslicing to
reorder passed inter-dependency hogs across the engines. However, we
require preemption to support timeslicing into user payloads, and not all
machine support preemption so we do not universally enable timeslicing,
even when it would correctly preempt our own inter-engine semaphores.
Since timeslicing and semaphore priority deboosting is now disabled on
Broadwell/Braswell, we have to follow suite and not use semaphores.

Testcase: igt/gem_exec_schedule/semaphore-codependency # bdw/bsw
Fixes: 18e4af04 ("drm/i915: Drop no-semaphore boosting")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200521140617.30015-1-chris@chris-wilson.co.uk