drm: remove drm_dev_to_irq from drivers

Only used in some legacy pci drivers, and dereferencing the PCI irq is
actually shorter ...

Since this removes all users for drm_dev_to_irq from the tree except
in drm_irq.c, move the inline helper in there. It'll disappear soon,
too.

v2: Polish commit message (Thierry)

Reviewed-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_irq.c

@@ -233,6 +233,11 @@ static void drm_irq_vgaarb_nokms(void *cookie, bool state)
 	}
 }
 
+static inline int drm_dev_to_irq(struct drm_device *dev)
+{
+	return dev->driver->bus->get_irq(dev);
+}
+
 /**
  * Install IRQ handler.
  *