agp: kill agp_flush_chipset and corresponding ioctl

The intel drm calls the chipset functions now directly. Userspace
never called the corresponding ioctl, hence it can be killed, too.

Cc: Dave Airlie <airlied@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>