drm/gma500: make function static to eliminate compiling warning

psb_gtt_remove is only used in this file, and make it static to
eliminate missing-prototypes compiling warning.

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/1477972185-24826-1-git-send-email-jiang.biao2@zte.com.cn