drm/i915: use direct alias for i915 in requests

i915_request contains direct alias to i915, there is no point to go via
rq->engine->i915.

v2: added missing rq.i915 initialization in measure_breadcrumb_dw.

Signed-off-by: Andrzej Hajda <andrzej.hajda@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Acked-by: Nirmoy Das <nirmoy.das@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230720113002.1541572-1-andrzej.hajda@intel.com