i965/vs: Make vec4_vs_visitor class derived from vec4_visitor.

i965/vs: Make vec4_vs_visitor class derived from vec4_visitor.

This patch just creates the derived class; later patches will migrate
VS-specific functions and data structures from the base class into the
derived class.

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