v3d: keep track of resources written by transform feedback

v3d: keep track of resources written by transform feedback

The hardware provides a feature to sync reads from previous transform feedback
writes in the same job so if we use this mechanism we no longer have to flush
the job.

In order to identify this scenario we need a mechanism to identify resources
that are written by transform feedback.

v2: use _mesa_pointer_set_create (Eric)

Reviewed-by: Eric Anholt <eric@anholt.net>