drm/radeon/dpm: add debugfs support for rv6xx

This allows you to look at the current DPM state via
debugfs.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/radeon_asic.c
Simple merge

diff --cc drivers/gpu/drm/radeon/radeon_asic.h
Simple merge

diff --cc drivers/gpu/drm/radeon/rv6xx_dpm.c
Simple merge