SO was always enabled before this change. That meant, after a call to
tu_CmdBindTransformFeedbackBuffersEXT to emit VPC_SO_BUFFER_SIZE, any
draw call (from the same render pass, in a different render pass, or in
a different cmdbuf) could potentially cause writes to the SO buffers
regardless of whether the draw is inside xfb begin/end or not.
I choose to emit VPC_SO_DISABLE instead of using stateobjs like
freedreno does only because it is simpler. It is not clear to me which
is more efficient to HW.
This also fixes double SO writes for gmem rendering. While
tu6_tile_render_begin was careful to disable SO for the draw pass,
tu6_emit_tile_select re-enabled it.
dEQP-VK.transform_feedback.* still passes. It fixes
dEQP-GLES3.functional.transform_feedback.* on angle.
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16502>
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