drm/amdkfd: expose pasid of secondary contexts by debugfs

drm/amdkfd: expose pasid of secondary contexts by debugfs

Current kfd debugfs interfaces only expose pasid
of the primary process, this commit exposes
pasid of secondary contexts by debugfs

Just like entries under sysfs,
the secondary contexts are named as
context_<id> under its primary kfd process.

The layout:
/sys/kernel/debug/kfd/proc# tree
.
└── 5802
    ├── context_0
    │   ├── pasid_1025
    │   └── pasid_63266
    ├── context_1
    │   ├── pasid_1025
    │   └── pasid_63266
    ├── pasid_1025
    └── pasid_63266

Another fix is, kfd_debugfs_add_process may fail,
this commit change it to return a meaningful
value other than void

Signed-off-by: Zhu Lingshan <lingshan.zhu@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>