drm/i915: Prevent dereference of engine before NULL check in error capture

smatch caught,
drivers/gpu/drm/i915/i915_gpu_error.c:1418 gem_record_rings() warn: variable dereferenced before check 'engine' (see line 1413)
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190621135246.20683-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gpu_error.c

@@ -1410,7 +1410,6 @@ static void gem_record_rings(struct i915_gpu_state *error)
 	for (i = 0; i < I915_NUM_ENGINES; i++) {
 		struct intel_engine_cs *engine = i915->engine[i];
 		struct drm_i915_error_engine *ee = &error->engine[i];
-		struct i915_ggtt *ggtt = engine->gt->ggtt;
 		struct i915_request *request;
 
 		ee->engine_id = -1;
@@ -1428,7 +1427,7 @@ static void gem_record_rings(struct i915_gpu_state *error)
 			struct i915_gem_context *ctx = request->gem_context;
 			struct intel_ring *ring;
 
-			ee->vm = ctx->vm ?: &ggtt->vm;
+			ee->vm = ctx->vm ?: &engine->gt->ggtt->vm;
 
 			record_context(&ee->context, ctx);