drm/amdkfd: Tidy up kfd_generate_gpu_id() uint64_t bitshift unpack

Dereference the one time and unpack the lower and upper 32bit
portions with the proper kernel helper macros.

Signed-off-by: Edward O'Callaghan <funfunctor@folklore1984.net>
Reviewed-by: Oded Gabbay <oded.gabbay@gmail.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>