drm/amdgpu: remove old CI DPM implementation

The power smu7 powerplay code is much more robust and has
been the default for a while now.  Remove the old code.

Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>