r600g,radeonsi: don't append to streamout buffers that haven't been used yet

r600g,radeonsi: don't append to streamout buffers that haven't been used yet

The FILLED_SIZE counter is uninitialized at the beginning, so we can't use it.
Instead, use offset = 0, which is what we always do when not appending.

This unexpectedly fixes spec/ARB_texture_multisample/sample-position/*.
Yes, the test does use transform feedback.

Cc: 10.3 10.4 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Glenn Kennard <glenn.kennard@gmail.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>