drm/i915/guc: Replace preempt_client lookup with engine->preempt_context

Circumvent the dance we currently perform to find the preempt_client and
lookup its HW context for this engine, as we know we have already pinned
the preempt_context on the engine.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190408091728.20207-15-chris@chris-wilson.co.uk