drm/amdkfd: Add procfs-style information for KFD processes

Add a folder structure to /sys/class/kfd/kfd/ called proc which contains
subfolders, each representing an active KFD process' PID, containing 1
file: pasid.

Signed-off-by: Kent Russell <kent.russell@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>