drm/amdgpu: add dpm helper function for DF Cstate control

The helper function hides software smu and legacy powerplay
implementation for DF Cstate control.

Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>