amdkfd: process pointer of a HIQ should be NULL

amdkfd: process pointer of a HIQ should be NULL

In kq_initialize, queue->process of a HIQ should
be NULL as initialized, because it does not belong
to any kfd_process.

This commit decommisions the function kfd_get_process() because
it can not locate a specific kfd_process among multiple
contexts and not any code path calls it after this commit.

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>