drm: omapdrm: Inline the pipe2vbl function

The function is only used in omap_irq.c and is just a wrapper around
dispc_mgr_get_vsync_irq(). Remove it and call the dispc function
directly.

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