doc-rst: Wire-up Backlight kernel-doc documentation

Include backlight so the documentation is now generated with make
htmldocs and friends.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/gpu/backlight.rst

+=================
+Backlight support
+=================
+
+.. kernel-doc:: drivers/video/backlight/backlight.c
+   :doc: overview
+
+.. kernel-doc:: include/linux/backlight.h
+   :internal:
+
+.. kernel-doc:: drivers/video/backlight/backlight.c
+   :export:

Documentation/gpu/index.rst

@@ -12,6 +12,7 @@ Linux GPU Driver Developer's Guide
    drm-uapi
    drm-client
    drivers
+   backlight
    vga-switcheroo
    vgaarbiter
    todo