OMAPDSS: HDMI: Split video_enable into video_enable/disable

To improve readability, split the video_enable HDMI IP operation
into two separate functions for enabling and disabling video.
The video_enable function is also modified to return an error value.

While there, update these operations for the OMAP4 IP accordingly.

Signed-off-by: Ricardo Neri <ricardo.neri@ti.com>
diff --git a/drivers/video/omap2/dss/dss_features.c b/drivers/video/omap2/dss/dss_features.c
index 3a28c5c..234ed8a 100644
--- a/drivers/video/omap2/dss/dss_features.c
+++ b/drivers/video/omap2/dss/dss_features.c
@@ -568,6 +568,7 @@
 	.pll_enable		=	ti_hdmi_4xxx_pll_enable,
 	.pll_disable		=	ti_hdmi_4xxx_pll_disable,
 	.video_enable		=	ti_hdmi_4xxx_wp_video_start,
+	.video_disable		=	ti_hdmi_4xxx_wp_video_stop,
 	.dump_wrapper		=	ti_hdmi_4xxx_wp_dump,
 	.dump_core		=	ti_hdmi_4xxx_core_dump,
 	.dump_pll		=	ti_hdmi_4xxx_pll_dump,