omap: rx51: Cleanup vdds_sdi supply construction

It is much more cleaner to use REGULATOR_SUPPLY macro and a device name
instead of having a reference to rx51_display_device.dev with #if defined()
guards.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>