drm: omapdrm: Simplify IRQ wait implementation

Now that the IRQ list is used for IRQ wait only we can merge
omap_drm_irq and omap_irq_wait and simplify the implementation.

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