drm: omapdrm: hdmi5: Allocate the omap_hdmi data structure dynamically

The omap_hdmi private data structure is currently stored as a global
variable. While no platform with multiple HDMI5 encoders currently
exists nor is planned, this doesn't comply with the kernel device model
and should thus be fixed.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>