drm/radeon: add Mullins dpm support.

Generic dpm support similar to Kabini. Mullins specific features
will be worked on later.

Signed-off-by: Samuel Li <samuel.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>