drm/amdgpu: Use active umc info from discovery

There could be configs where some UMC instances are harvested. This
information is obtained through discovery data and populated in
umc.active_mask. Avoid reassigning this as AID mask, instead use the
mask directly while iterating through umc instances. This is to avoid
accesses to harvested UMC instances.

v2: fix warning (Alex)

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>