drm: merge drm_memory.h into drm_memory.c

The drm_memory.h header is only used to define PAGE_AGP, which is only
used in drm_memory.c. Fold the header into drm_memory.c and drop it.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>