i965: Rename backend_visitor::prog to shader_prog.

i965: Rename backend_visitor::prog to shader_prog.

The next patch is going to change the type of vec4_visitor::vp from
struct gl_vertex_program * to struct gl_program *, and rename it.  The
sensible name to change it to is vec4_visitor::prog.  However, prog is
already used in backend_visitor (which vec4_visitor derives from).
Since backend_visitor::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>