drm: omapdrm: Don't expose the omap_irq_(un)register() functions

The IRQ registration functions are not used outside of their compilation
unit, make them static. As the __omap_irq_(un)register() functions are
only called by their omap_irq_(un)register() counterparts, merge them
together.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>