coreboot: Fix brightness controls

The `gfx` register got dropped during some rebase. Add them back.

Fixes brightness controls on Windows for oryp8, gaze16, lemp9.

Signed-off-by: Tim Crawford <tcrawford@system76.com>

CHANGELOG.md

@@ -4,6 +4,10 @@ Changes are identified by the date of the released firmware including them. If
 you are running System76 Open Firmware, opening the boot menu will show this
 date followed by an underscore and a short git revision.
 
+## unreleased
+
+- oryp8: Fixed brightness controls on Windows
+
 ## 2022-09-07
 
 - Updated CSME for TGL-H to 15.0.41.2158