amdkfd: fence handler evict and restore a kfd process by its context id

amdkfd: fence handler evict and restore a kfd process by its context id

In fence enable signaling handler, kfd evicts
and restores the corresponding kfd_process,
this commit helps find the kfd_process by
both its mm and context id.

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>