drm/amd/display: update bios scratch when setting backlight

Update the bios scratch register when updating the backlight
level.  Some platforms apparently read this scratch register
and do additional operations in their hotkey handlers.

Bug: https://gitlab.freedesktop.org/drm/amd/-/issues/1518


Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>