amdkfd: process USERPTR allocation only on the primary kfd process

amdkfd: process USERPTR allocation only on the primary kfd process

The lifecycle of the primary kfd process is tied to
the user space program, all secondary kfd process
would be destroyed when fd close. Thus only the primary
kfd process should process USERPTR memory allocation.

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>