drm/amdgpu: Refactor XGMI reset on init handling

Use XGMI hive information to rely on resetting XGMI devices on
initialization rather than using mgpu structure. mgpu structure may have
other devices as well.

Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Feifei Xu <feifxu@amd.com>
Acked-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Tested-by: Rajneesh Bhardwaj <rajneesh.bhardwaj@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>