i965: Pull stage_prog_data.nr_params out of the NIR shader

i965: Pull stage_prog_data.nr_params out of the NIR shader

Previously, we had a bunch of code in each stage to figure out how many
slots we needed in stage_prog_data.param.  This code was mostly identical
across the stages and had been copied and pasted around.  Unfortunately,
this meant that any time you did something special, you had to add code for
it to each of these places.  In particular, none of the stages took
subroutines into account; they were working entirely by accident.  By
taking this data from the NIR shader, we know the exact number of entries
we need and everything goes a bit smoother.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>