drm: omapdrm: Remove unused parameter from omap_drm_irq handler

The only omap_drm_irq handler doesn't use the irqstatus parameter passed
to the function. Remove it.

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