anv: fix xfb usage with rasterizer discard

anv: fix xfb usage with rasterizer discard

in the initial implementation, a stream like:

* CmdBeginTransformFeedbackEXT
* CmdSetRasterizerDiscardEnableEXT
* CmdDraw
* CmdEndTransformFeedbackEXT
* CmdBeginTransformFeedbackEXT
* CmdDraw
* CmdEndTransformFeedbackEXT

would never enable transform feedback, as it only checked for the change
in rasterizer_discard state

Fixes: 4d531c67dfd ("anv: support rasterizer discard dynamic state")

Reviewed-by: Jason Ekstrand <jason.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15269>