drm/i915/buddy: avoid double list_add

Be careful not to mark an already free node as free again.
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200305204711.217783-1-matthew.auld@intel.com

drivers/gpu/drm/i915/i915_buddy.c

@@ -312,7 +312,8 @@ i915_buddy_alloc(struct i915_buddy_mm *mm, unsigned int order)
 	return block;
 
 out_free:
-	__i915_buddy_free(mm, block);
+	if (i != order)
+		__i915_buddy_free(mm, block);
 	return ERR_PTR(err);
 }