nv50: don't negate immediates in set_immd

nv50: don't negate immediates in set_immd

This negation would only be triggered in situations
where it's incorrect.
The caller of set_immd should negate the immediate value
in the instruction itself if desired, and will also know
if it's a float or an int.

ADD TEMP[0], CONST[0], -IMMD[0] would load the immediate
into extra TEMP, negated, and set the negate flag in add
as well - double negation.