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/index.rst

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