i965 gen6: Ensure correct transform feedback indices on new batch.

i965 gen6: Ensure correct transform feedback indices on new batch.

We don't currently have kernel support for saving GPU registers on a
context switch, so if multiple processes are performing transform
feedback at the same time, their SVBI registers will interfere with
each other.  To avoid this situation, we keep a software shadow of the
state of the SVBI 0 register (which is the only register we use), and
re-upload it on every new batch.

The function that updates the shadow state of SVBI 0 is called
brw_update_primitive_count, since it will also be used to update the
counters for the PRIMITIVES_GENERATED and
TRANSFORM_FEEDBACK_PRIMITIVES_WRITTEN queries.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>