OMAPDSS: HDMI: use vdda_hdmi_dac

The HDMI driver requires vdda_hdmi_dac power for operation, but does not
enable it. This has worked because the regulator has been always
enabled.

But this may not always be the case, as I encountered when implementing
HDMI device tree support.

This patch changes the HDMI driver to use the vdda_hdmi_dac.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>