gallium: replace prim pipeline begin/end() functions with flush()

gallium: replace prim pipeline begin/end() functions with flush()

This is basically half of Keith's draw/flush patch.

The stage->point/line/tri() functions are now self-validating, the validator
functions are installed by the flush() function.

There were excessive calls to validate_pipeline(), however.  This was caused
by draw_prim_queue_flush() keeping a local 'first' variable that always pointed
to the validate functions.  Replaced 'first' with 'draw->pipeline.first'.

Performance in gears is up just slightly with this patch.