drm/radeon/dpm: add debugfs support for RS780/RS880 (v3)

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

Due to the way the hardware works on these asics, there's
no way to look up exactly what power state we are in, so
we make the best guess we can based on the current sclk.

v2: Anthoine's version
v3: fix ref div

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