drm/i915: BUG_ON put_pages later

With multiple VMs, the eviction code benefits from being able to blindly
put pages without needing to know if there are any entities still
holding on to those pages. As such it's preferable to return the -EBUSY
before the BUG.

Eviction code is the only user for now, but overall it makes sense
anyway, IMO.
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -1668,11 +1668,11 @@ i915_gem_object_put_pages(struct drm_i915_gem_object *obj)
 	if (obj->pages == NULL)
 		return 0;
 
-	BUG_ON(i915_gem_obj_ggtt_bound(obj));
-
 	if (obj->pages_pin_count)
 		return -EBUSY;
 
+	BUG_ON(i915_gem_obj_ggtt_bound(obj));
+
 	/* ->put_pages might need to allocate memory for the bit17 swizzle
 	 * array, hence protect them from being reaped by removing them from gtt
 	 * lists early. */