drm/memory: don't export agp helpers

They're only used by the agpgart support code in drm_agpgart.c,
not by any drivers.

I think long-term we should create a drm_internal.h include file with
all the various functions only used by the drm core and not exported
to drivers, and remove them from drmP.h. Oh, and someone should kill
that upper-case P sometimes ;-) But that's all stuff for future patch
bombs.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>