i965/fs: Take into account copy register offset during compute-to-mrf.

i965/fs: Take into account copy register offset during compute-to-mrf.

This was dropping 'inst->dst.offset' on the floor.  Nothing in the
code above seems to guarantee that it's zero and in that case the
offset of the register being coalesced into wouldn't be taken into
account while rewriting the generating instruction.

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