drm/i915: Fix a use-after-free in intel_execlists_retire_requests

Remove request from list before unreferencing it, in case it's actually
the only reference. (Found by Tvrtko Ursulin)

This issue has been most likely introduced in

commit 6d3d8274
Author: Nick Hoath <nicholas.hoath@intel.com>
Date:   Thu Jan 15 13:10:39 2015 +0000

    drm/i915: Subsume intel_ctx_submit_request in to drm_i915_gem_request
Signed-off-by: Nick Hoath <nicholas.hoath@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_lrc.c

@@ -732,8 +732,8 @@ void intel_execlists_retire_requests(struct intel_engine_cs *ring)
 			intel_lr_context_unpin(ring, ctx);
 		intel_runtime_pm_put(dev_priv);
 		i915_gem_context_unreference(ctx);
-		i915_gem_request_unreference(req);
 		list_del(&req->execlist_link);
+		i915_gem_request_unreference(req);
 	}
 }