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>

include/drm/drmP.h

@@ -1185,11 +1185,6 @@ static __inline__ int drm_core_check_feature(struct drm_device *dev,
 	return ((dev->driver->driver_features & feature) ? 1 : 0);
 }
 
-static inline int drm_dev_to_irq(struct drm_device *dev)
-{
-	return dev->driver->bus->get_irq(dev);
-}
-
 static inline void drm_device_set_unplugged(struct drm_device *dev)
 {
 	smp_wmb();