i965: Move vec4 register allocation data structures to brw->vec4.

i965: Move vec4 register allocation data structures to brw->vec4.

This will avoid confusion when we add geometry shaders, since these
data structures will be shared by vertex and geometry shaders.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>