drm/sis|via: don't return stack garbage from free_mem ioctl

Fallout from my "kill drm_sman" refactor. Unfortunately gcc seems to
have failed me and not warned about this.

Tested-by: Jakob Bornecrantz <wallbraker@gmail.com> (on via)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>