drm/i915/dg2: s/engine->i915/i915/ for engine workarounds

rcs_engine_wa_init() has a local 'i915' variable; we should use that
rather than 'engine->i915' for consistency with how we handle other
platforms.

Suggested-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220128170125.4121819-1-matthew.d.roper@intel.com