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

drivers/gpu/drm/gma500/gtt.c

@@ -131,7 +131,7 @@ static int psb_gtt_insert(struct drm_device *dev, struct gtt_range *r,
  *	page table entries with the dummy page. This is protected via the gtt
  *	mutex which the caller must hold.
  */
-void psb_gtt_remove(struct drm_device *dev, struct gtt_range *r)
+static void psb_gtt_remove(struct drm_device *dev, struct gtt_range *r)
 {
 	struct drm_psb_private *dev_priv = dev->dev_private;
 	u32 __iomem *gtt_slot;