i965/vs: Make type of vec4_visitor::vp more generic.

i965/vs: Make type of vec4_visitor::vp more generic.

The vec4_visitor functions don't use any VS specific data from
vec4_visitor::vp.  So rename it to "prog" and change its type from
struct gl_vertex_program * to struct gl_program *.  This will allow
the code to be re-used 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>