amdkfd: find_process_by_mm always return the primary context

amdkfd: find_process_by_mm always return the primary context

Up until this commit, the kfd multiple contexts feature has
not been fully implemented in mainline kernel yet.

For backawrd compatibility, not break existing use cases,
this commit changes function find_process_by_mm, let it
always return the primary kfd_process.

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>