broadcom/compiler: add small_imm a/c/d on v3d_qpu_sig

broadcom/compiler: add small_imm a/c/d on v3d_qpu_sig

small_imm_a, small_imm_c and small_imm_d added on top of the already
existing small_imm_b, as V3D 7.1 defines 4 small immediates, tied to
the 4 raddr. Note that this is only the definition, and just a inst
validation rule to check that are not used before v71. Any real use is
still pending.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/25450>