drm/sun4i: Make some symbols static

Fix sparse warnings:

drivers/gpu/drm/sun4i/sun8i_tcon_top.c:271:36: warning: symbol 'sun8i_r40_tcon_top_quirks' was not declared. Should it be static?
drivers/gpu/drm/sun4i/sun8i_tcon_top.c:276:36: warning: symbol 'sun50i_h6_tcon_top_quirks' was not declared. Should it be static?
drivers/gpu/drm/sun4i/sun4i_tcon.c:239:6: warning: symbol 'sun4i_tcon_set_mux' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190416145855.20852-1-yuehaibing@huawei.com