drm: bridge/dw_hdmi: add function dw_hdmi_phy_enable_spare

RK3288 HDMI will not work without the spare bit of
HDMI_PHY_CONF0 enable

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Tested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>