OMAPDSS: DISPC: Remove boolean comparisons

Boolean tests do not need explicit comparison to true or false.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>