radv: apply xfb buffer offset at buffer binding time not later. (v2)

radv: apply xfb buffer offset at buffer binding time not later. (v2)

In order to handle pause/resume properly, the offset should
be added to the buffer binding not to the begin/end paths.

v2: don't add offset to size
Fixes ext_transform_feedback-alignment* under zink

Fixes: b4eb029062 (radv: implement VK_EXT_transform_feedback)
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>