ARM: OMAP: Sleep is prevented when no LCD is attached

We have to make sure that the LCD DMA external destination bit is
cleared by default, otherwise OMAP won't sleep.

Signed-off-by: Imre Deak <imre.deak@solidboot.com>
Signed-off-by: Juha Yrjola <juha.yrjola@solidboot.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>