i965/vs: Make some vec4_visitor functions virtual.

i965/vs: Make some vec4_visitor functions virtual.

This patch makes the following vec4_visitor functions virtual, since
they will need to be implemented differently for vertex and geometry
shaders.  Some of the functions are renamed to reflect their generic
purpose, rather than their VS-specific behaviour:

- setup_attributes
- emit_attribute_fixups (renamed to emit_prolog)
- emit_vertex_program_code (renamed to emit_program_code)
- emit_urb_writes (renamed to emit_thread_end)

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>