r600g/sb: fix handling of preloaded inputs for compute shaders

For compute shaders we need to let the backend know that
GPRs 0 and 1 are preloaded with some compute-specific input
values, otherwise any use of these regs without previous
definition is considered as undefined value and usually
is simply replaced with 0.

Signed-off-by: Vadim Girlin <vadimgirlin@gmail.com>