intel: Rename plane[AB]* back to pipe[AB]*.

The values are really going to continue meaning pipe, not plane, and that's
what they're called in the kernel copy of the header.  Userland hasn't ever
made the switch to pipe!=plane, since userland checks are based on DRM
version, which is still stuck at 1.6.  However, Mesa did start using
plane[AB] names, so provide a compat define.