r600g: rework state emission of vertex buffers

r600g: rework state emission of vertex buffers

This reduces a little of CPU overhead.
The idea is to translate pipe vertex buffers directly into the CS
and not using any intermediate representations.

Framerate in Torcs:
  before: 32.2
  after:  34.6

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>