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: 4d531c67 ("anv: support rasterizer discard dynamic state")