drm/gma500: remove unused ioctl declarations

psb_gem_create_ioctl and psb_gem_mmap_ioctl are not used currently,
their declarations are useless.

Signed-off-by: Jiang Biao <jiang.biao2@zte.com.cn>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1477990879-13139-1-git-send-email-jiang.biao2@zte.com.cn