i965/vs: Generalize data structures pointed to by vec4_generator.

i965/vs: Generalize data structures pointed to by vec4_generator.

This patch removes the following field from vec4_generator, since it
is not used:

- struct brw_vs_compile *c

And changes the following field:

- struct gl_vertex_program *vp => struct gl_program *prog

With these changes, vec4_generator no longer refers to any VS-specific
data structures.  This will pave the way for re-using it for geometry
shaders.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
v2: Use the name "prog" rather than "p".

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>