drm/i915: Replace __I915__ with typesafe variant

Ville pointed out the GCCism __builtin_types_compatible_p() that we
could use to replace our heavily casted presumption __I915__ macro that
was based on comparing struct sizes.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_dma.c

@@ -1595,9 +1595,6 @@ int i915_driver_load(struct drm_device *dev, unsigned long flags)
 	if (!drm_core_check_feature(dev, DRIVER_MODESET) && !dev->agp)
 		return -EINVAL;
 
-	/* For the ugly agnostic INTEL_INFO macro */
-	BUILD_BUG_ON(sizeof(*dev_priv) == sizeof(*dev));
-
 	dev_priv = kzalloc(sizeof(*dev_priv), GFP_KERNEL);
 	if (dev_priv == NULL)
 		return -ENOMEM;

drivers/gpu/drm/i915/i915_drv.h

@@ -2004,8 +2004,16 @@ struct drm_i915_cmd_table {
 };
 
 /* Note that the (struct drm_i915_private *) cast is just to shut up gcc. */
-#define __I915__(p)	((sizeof(*(p)) == sizeof(struct drm_i915_private)) ? \
-			 (struct drm_i915_private *)(p) : to_i915(p))
+#define __I915__(p) ({ \
+	struct drm_i915_private *__p; \
+	if (__builtin_types_compatible_p(typeof(*p), struct drm_i915_private)) \
+		__p = (struct drm_i915_private *)p; \
+	else if (__builtin_types_compatible_p(typeof(*p), struct drm_device)) \
+		__p = to_i915((struct drm_device *)p); \
+	else \
+		BUILD_BUG(); \
+	__p; \
+})
 #define INTEL_INFO(p) 	(&__I915__(p)->info)
 #define INTEL_DEVID(p)	(INTEL_INFO(p)->device_id)