fbdev: sh_mobile_hdmi: enable "external" mode

The SH-Mobile HDMI controller supports two configuration modes: using
pre-programmed VICs and the "external" mode - specifying video parameters
explicitly. The driver already contains code, necessary to configure HDMI
manually, this patch actually enables it.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>