i965/vs: Rename vec4_generator::prog to shader_prog.

i965/vs: Rename vec4_generator::prog to shader_prog.

The next patch is going to change the type of vec4_generator::vp from
struct gl_vertex_program * to struct gl_program *, and rename it.  The
sensible name to change it to is vec4_generator::prog.  However, prog
is already used.  Since the existing vec4_generator::prog is of type
struct gl_shader_program, it makes sense to rename it to shader_prog.

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