drm/i915: fix warning for unused variable

drivers/gpu/drm/i915/intel_engine_cs.c: In function ‘intel_engine_is_idle’:
drivers/gpu/drm/i915/intel_engine_cs.c:1103:27: error: unused variable ‘dev_priv’ [-Werror=unused-variable]
  struct drm_i915_private *dev_priv = engine->i915;
                           ^~~~~~~~

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>