i965/vec4: Move c->last_scratch into vec4_visitor.

i965/vec4: Move c->last_scratch into vec4_visitor.

Nothing outside of vec4_visitor uses it, so we may as well keep it
internal.

Commit db9c915abcc5ad78d2d11d0e732f04cc94631350 for the vec4 backend.

(The empty class will be going away soon.)

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>