drm/arc: Fix some sparse warnings

Fixes the following sparse warnings:

drivers/gpu/drm/arc/arcpgu_drv.c:52:5: warning:
  symbol 'arcpgu_gem_mmap' was not declared. Should it be static?
drivers/gpu/drm/arc/arcpgu_drv.c:134:48: warning:
  Using plain integer as NULL pointer
drivers/gpu/drm/arc/arcpgu_drv.c:155:5: warning:
  symbol 'arcpgu_unload' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Sean Paul <seanpaul@chromium.org>

drivers/gpu/drm/arc/arcpgu_drv.c

@@ -48,7 +48,7 @@ static void arcpgu_setup_mode_config(struct drm_device *drm)
 	drm->mode_config.funcs = &arcpgu_drm_modecfg_funcs;
 }
 
-int arcpgu_gem_mmap(struct file *filp, struct vm_area_struct *vma)
+static int arcpgu_gem_mmap(struct file *filp, struct vm_area_struct *vma)
 {
 	int ret;
 
@@ -130,7 +130,7 @@ static int arcpgu_load(struct drm_device *drm)
 		if (ret < 0)
 			return ret;
 	} else {
-		ret = arcpgu_drm_sim_init(drm, 0);
+		ret = arcpgu_drm_sim_init(drm, NULL);
 		if (ret < 0)
 			return ret;
 	}
@@ -151,7 +151,7 @@ static int arcpgu_load(struct drm_device *drm)
 	return 0;
 }
 
-int arcpgu_unload(struct drm_device *drm)
+static int arcpgu_unload(struct drm_device *drm)
 {
 	struct arcpgu_drm_private *arcpgu = drm->dev_private;