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>