drm/amd/powerplay: hold on the arcturus gfx dpm support in driver

As for now, only "Prefetcher" is guarded to be working from
SMU firmware.

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