amdgpu/powerplay: constify large struct

This moves this from being global data to global rodata, I'm
sure it would be easy to move it to being local data.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>