omapfb/displays: change CONFIG_DISPLAY_* to CONFIG_FB_OMAP2_*

We need to change the config symbols of omapfb's private copy of the
panel and encoder drivers so that we won't have config symbol conflicts.

This patch changes the symbols from the panel and encoder drivers using
simple replacement of DISPLAY_* to FB_OMAP2*.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Dave Airlie <airlied@gmail.com>
Acked-by: Rob Clark <robdclark@gmail.com>