linuxkpi: Define backlight_get_brightness() and backlight_is_blank()

This is not used by the DRM driver yet because we comment out the code
calling them, but they are easy to implement.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38530

(cherry picked from commit a82a8a5e193587d819288c3c27f1c9472178e10e)