drm/amdkfd: Return NULL if kfd_lookup_process_by_pasid fails

If no matching process is found, return NULL instead of a pointer
to the last process in the kfd_processes_table.

Signed-off-by: Yong Zhao <yong.zhao@amd.com>
Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>