drm: remove unused "struct drm_freelist"

This object is not used except for static fields in drm_bufs *cough*.
Inline the watermark fields and drop the unused structure definition.

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>