a3xx: fix rel/const/rel-const register encoding

In most cases a3xx seems to actually use a scheme whereby it can encode
an extra bit for const register.  You have three possible encodings in
thirteen bits:

  register:  (11 bits for N.c)
    00........... rN.c

  relative:  (10 bits for N)
    010.......... r<a0.x + N>
    011.......... c<a0.x + N>

  const:     (12 bits for N.c)
    1............ cN.c