drm/amd/powerplay: delete some dead code

We deleted some code in e154162e ("drm/amd/powerplay: refine pp code
for raven") but there were a few related bits that were missed.  Let's
delete them as well.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>