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>