drm/i915: Remove redundant WARN_ON from __i915_add_request()

It's an outright programming error, so explode if it is ever hit.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1471254551-25805-7-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem_request.c

@@ -470,18 +470,12 @@ static void i915_gem_mark_busy(const struct intel_engine_cs *engine)
  */
 void __i915_add_request(struct drm_i915_gem_request *request, bool flush_caches)
 {
-	struct intel_engine_cs *engine;
-	struct intel_ring *ring;
+	struct intel_engine_cs *engine = request->engine;
+	struct intel_ring *ring = request->ring;
 	u32 request_start;
 	u32 reserved_tail;
 	int ret;
 
-	if (WARN_ON(!request))
-		return;
-
-	engine = request->engine;
-	ring = request->ring;
-
 	/*
 	 * To ensure that this call will not fail, space for its emissions
 	 * should already have been reserved in the ring buffer. Let the ring