drm/amd/amdgpu: hide voltage and power sensors on SI and KV parts

The driver does not support these sensors yet and there is no point in
creating sysfs attributes which will always return an error.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "David (ChunMing) Zhou" <David1.Zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>