radeonsi: use shifts for sign extension

radeonsi: use shifts for sign extension

Avoids a branch and reduces code size a tiny bit:
    text   data     bss      dec    hex filename
10804563 398653 2070368 13273584 ca89f0 /tmp/radeonsi_dri.so.old
10804499 398653 2070368 13273520 ca89b0 /tmp/radeonsi_dri.so

Reviewed-by: Marek Olšák <marek.olsak@amd.com>