radv/xfb: don't increase offset by component mask start.

This is incorrect, the offset is into the buffer, and it's legal
to write

loc 0,0 -> buffer0, offset 0
loc 0,1 -> buffer1, offset 0

This fixes a bunch of piglits running on my zink xfb code on
radv.

Fixes: 6c21645046 (radv: emit stream outputs for vertex and tessellation stages)
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>