drm/amd/pm: Mark MM activity as unsupported

Aldebaran doesn't support querying MM activity percentage. Keep the
field as 0xFFs to mark it as unsupported.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org