agp: kill agp_(unmap|map)_memory

DMA remapping was only used by the intel-gtt driver. With that
code now folded into the driver, kill the agp generic support for
it.

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