This patch does two things:

1. Constant buffer state changes were broken (but happened to work by
   dumb luck).  The problem is we weren't calling draw_do_flush() in
   draw_set_mapped_constant_buffer() when we changed that state.  All the
   other draw_set_foo() functions were calling draw_do_flush() already.

2. Use a simpler state validation step when we're changing light-weight
   parameter state such as constant buffers, viewport dims or clip planes.
   There's no need to revalidate the whole pipeline when changing state
   like that.  The new validation method is called bind_parameters()
   and is called instead of the prepare() method.  A new
   DRAW_FLUSH_PARAMETER_CHANGE flag is used to signal these light-weight
   state changes.  This results in a modest but measurable increase in
   FPS for many Mesa demos.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>