doc-rst: include index only in subproject AND html

The index should only be included if the build of the sub-folder is done
with the html-builder (HTML output).

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/gpu/index.rst b/Documentation/gpu/index.rst
index be0dafc..367d7c3 100644
--- a/Documentation/gpu/index.rst
+++ b/Documentation/gpu/index.rst
@@ -14,7 +14,7 @@
    vga-switcheroo
    vgaarbiter
 
-.. only::  subproject
+.. only::  subproject and html
 
    Indices
    =======