drm/irq: simplify irq checks in drm_wait_vblank

Checking for both an irq number _and_ whether it's enabled is
redundant. Originally I've thought the drm_dev_to_irq call would break
drivers which do their own irq checking, but those shouldn't have
DRIVER_HAVE_IRQ set as Thierry Reding pointed out. But such drivers
already need to set dev->irq_enabled for other reasons, so we might as
well ditch that check, too.

v2: Also drop the HAVE_IRQ check.

Cc: Thierry Reding <thierry.reding@gmail.com>
Reviewed-by: NThierry Reding <treding@nvidia.com>
Reviewed-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_irq.c

@@ -1160,9 +1160,8 @@ int drm_wait_vblank(struct drm_device *dev, void *data,
 	int ret;
 	unsigned int flags, seq, crtc, high_crtc;
 
-	if (drm_core_check_feature(dev, DRIVER_HAVE_IRQ))
-		if ((!drm_dev_to_irq(dev)) || (!dev->irq_enabled))
-			return -EINVAL;
+	if (!dev->irq_enabled)
+		return -EINVAL;
 
 	if (vblwait->request.type & _DRM_VBLANK_SIGNAL)
 		return -EINVAL;