i965/fs: Actually set/use the mlen for gen7 uniform pull constant loads

i965/fs: Actually set/use the mlen for gen7 uniform pull constant loads

Previously, we were allocating the payload with different sizes per gen and
then figuring out the mlen in the generator based on gen.  This meant,
among other things, that the higher level passes knew nothing about it.

Acked-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>