platform/x86: Add missing LPS0 functions for AMD

These are supposedly not required for AMD platforms,
but at least some HP laptops seem to require it to
properly turn off the keyboard backlight.

Based on a patch from Marcin Bachry <hegel666@gmail.com>.

Bug: drm/amd#1230


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Marcin Bachry <hegel666@gmail.com>