amdkfd: identify a secondary kfd process by its id

amdkfd: identify a secondary kfd process by its id

This commit introduces a new id field for
struct kfd process, which helps identify
a kfd process among multiple contexts that
all belong to a single user space program.

The sysfs entry of a secondary kfd process
is placed under the sysfs entry folder of
its primary kfd process.

The naming format of the sysfs entry of a secondary
kfd process is "context_%u" where %u is the 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>