drm/amdkfd: Get reference to lead_thread task struct

Increment the kfd_process.lead_thread's reference counter to make
it safe to dereference. This is needed for getting a safe reference
to the process' mm_struct.

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>