drm/i915: Avoid allocation for execbuffer object list

Besides the minimal improvement in reducing the execbuffer overhead, the
real benefit is clarifying a few routines.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index b30c6c1..d9d81f9 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3399,6 +3399,7 @@
 	INIT_LIST_HEAD(&obj->mm_list);
 	INIT_LIST_HEAD(&obj->gtt_list);
 	INIT_LIST_HEAD(&obj->ring_list);
+	INIT_LIST_HEAD(&obj->exec_list);
 	INIT_LIST_HEAD(&obj->gpu_write_list);
 	obj->madv = I915_MADV_WILLNEED;
 	/* Avoid an unnecessary call to unbind on the first bind. */