drm: Allow vblank support without DRIVER_HAVE_IRQ

Drivers that register interrupt handlers without the DRM core helpers
don't initialize the .irq_enabled field and drm_dev_to_irq() may fail
when called on them. This shouldn't preclude them from implementing
the vblank IOCTL.
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_irq.c

@@ -1218,6 +1218,7 @@ 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;