i965: Introduce a "render ring prelude" hook.

i965: Introduce a "render ring prelude" hook.

The new intel_batchbuffer_emit_render_ring_prelude() hook will be called
when switching from BLT or UNKNOWN_RING to RENDER_RING.  This provides a
place to emit state that should go at the start of each render ring
batch, with minimal overhead.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>