i965/fs: Fix bogus sub-MRF offset calculation in compute-to-mrf.

i965/fs: Fix bogus sub-MRF offset calculation in compute-to-mrf.

The 'scan_inst->dst.offset % REG_SIZE' term in the final
'scan_inst->dst.offset' calculation is obviously bogus.  The offset
from the start of the copy destination register 'inst->dst' where the
destination of the generating instruction 'scan_inst' would be written
to (before compute-to-mrf runs) is just the offset of 'scan_inst->dst'
relative to the source of the copy instruction (AKA rel_offset in the
code below).

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