i965/vs: Move some functions from brw_vec4_emit.cpp to brw_vec4.cpp.

i965/vs: Move some functions from brw_vec4_emit.cpp to brw_vec4.cpp.

This leaves only the final code generation stage in brw_vec4_emit.cpp,
moving the payload setup, run(), and brw_vs_emit functions to brw_vec4.cpp.

The fragment shader backend puts these functions in brw_fs.cpp, so this
patch also helps with consistency.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>