drm/amd/display: Implement support for backlight optimization

- Add functionality to get real hw backlight level as opposed to user
level, meaning the level that takes into account backlight ramping
over time and backlight reduction due to Varibright
- Add backlight optimization which allows for a second OS state
that is able to control ABM

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>