drm/omap: fix incorrect union usage

The DSI encoder sets dssdev->ops->dsi.set_config, which is stored at the
same offset as dssdev->ops->hdmi.set_hdmi_mode. The code in omap_encoder
only checks if dssdev->ops->hdmi.set_hdmi_mode is NULL. Due to the way
union works, it won't be NULL if dsi.set_config is set. This means
dsi_set_config will be called with config=hdmi_mode=false=NULL parameter
resulting in a NULL dereference. Also the dereference happens while
console is locked, so kernel hangs without any debug output without
"fb.lockless_register_fb=1" parameter.

This restructures the code, so that the HDMI mode is only configured
for HDMI output types.

Fixes: 83910ad3

 ("drm/omap: Move most omap_dss_driver operations to omap_dss_device_ops")
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Tested-by: Tony Lindgren <tony@atomide.com>
[tomi.valkeinen@ti.com: dropped the safeguard]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181121160916.22017-5-sebastian.reichel@collabora.com