drm/i915: make i915_utils.h self-contained

And ensure it stays that way.
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/69bcebefa6d8689d4a962394b0c6db04904354ed.1556809195.git.jani.nikula@intel.com

drivers/gpu/drm/i915/Makefile.header-test

@@ -14,6 +14,7 @@ header_test := \
 	i915_reg.h \
 	i915_scheduler_types.h \
 	i915_timeline_types.h \
+	i915_utils.h \
 	intel_acpi.h \
 	intel_atomic.h \
 	intel_atomic_plane.h \

drivers/gpu/drm/i915/i915_utils.h

@@ -25,6 +25,10 @@
 #ifndef __I915_UTILS_H
 #define __I915_UTILS_H
 
+#include <linux/list.h>
+#include <linux/types.h>
+#include <linux/workqueue.h>
+
 #undef WARN_ON
 /* Many gcc seem to no see through this and fall over :( */
 #if 0
@@ -152,8 +156,6 @@ static inline u64 ptr_to_u64(const void *ptr)
 	__idx;								\
 })
 
-#include <linux/list.h>
-
 static inline void __list_del_many(struct list_head *head,
 				   struct list_head *first)
 {