drm/doc: Group the fb gem helpers better

Instead of spreading them all over the place.

Cc: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709084016.23750-13-daniel.vetter@ffwll.ch

Documentation/gpu/drm-kms-helpers.rst

@@ -109,6 +109,15 @@ Framebuffer CMA Helper Functions Reference
 
 .. _drm_bridges:
 
+Framebuffer GEM Helper Reference
+================================
+
+.. kernel-doc:: drivers/gpu/drm/drm_gem_framebuffer_helper.c
+   :doc: overview
+
+.. kernel-doc:: drivers/gpu/drm/drm_gem_framebuffer_helper.c
+   :export:
+
 Bridges
 =======
 
@@ -291,15 +300,6 @@ Auxiliary Modeset Helpers
 .. kernel-doc:: drivers/gpu/drm/drm_modeset_helper.c
    :export:
 
-Framebuffer GEM Helper Reference
-================================
-
-.. kernel-doc:: drivers/gpu/drm/drm_gem_framebuffer_helper.c
-   :doc: overview
-
-.. kernel-doc:: drivers/gpu/drm/drm_gem_framebuffer_helper.c
-   :export:
-
 Legacy Plane Helper Reference
 =============================