ARM: OMAP2+: Fix LCD panel backlight regression for LDP legacy booting

Looks like the LCD panel on LDP has been broken quite a while, and
recently got fixed by commit 0b2aa8be (gpio: twl4030: Fix regression
for twl gpio output). However, there's still an issue left where the panel
backlight does not come on if the LCD drivers are built into the
kernel.

Fix the issue by registering the DPI LCD panel only after the twl4030
GPIO has probed.

Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
[tony@atomide.com: updated per Tomi's comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>