drm/i915/execlists: Select arb on/off around batches based on preemption

Decide whether or not we need to disable arbitration within user batches
based on our intel_engine_has_preemption() flag.

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/20191213151331.1788371-1-chris@chris-wilson.co.uk