ARM/video: omap2: Move omap_display_init declaration to mach-omap2/display.h

The omap_display_init() is implemented in the mach-omap2/display.c so the
declaration should have been there as well.
Change the board files to include display.h to avoid build breakage at the
same time.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>