drm/ast: Make ast_primary_plane_helper_atomic_update static

Fix the following sparse warning:

drivers/gpu/drm/ast/ast_mode.c:564:6: warning:
symbol 'ast_primary_plane_helper_atomic_update'
was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Samuel Zou <zou_wei@huawei.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/1588819206-11406-1-git-send-email-zou_wei@huawei.com