drm/radeon/dpm: implement force performance levels for rs780 (v2)

Allows you to limit the selected power levels via sysfs.

Force the feedback divider to select a power level.

v2: fix checking in rs780_force_fbdiv,
    drop a duplicate divider structure in rs780_dpm_force_performance_level,
    Force the voltage level too.

Signed-off-by: Anthoine Bourgeois <anthoine.bourgeois@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>