drm/omap: remove read_irqenable()

We only use read_irqenable() to flush posted write. Instead of having a
separate function for this, do the flush implicitly in write_irqenable().
Thus we can remove read_irqenable().

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