drm/vblank: Lock down vblank->hwmode more

In the previous patch we've implemented hwmode tracking a la i915 for
the vblank timestamp calculations. But that was just the basic
semantics, i915 has some nice sanity checks to make sure we keep
getting this right. Move them over too.

v2:
- WARN_ON_ONCE to avoid excessive spam (Ville)
- Really only WARN on atomic drivers.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170509140329.24114-5-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_irq.c

@@ -777,6 +777,8 @@ bool drm_calc_vbltimestamp_from_scanoutpos(struct drm_device *dev,
 	 */
 	if (mode->crtc_clock == 0) {
 		DRM_DEBUG("crtc %u: Noop due to uninitialized mode.\n", pipe);
+		WARN_ON_ONCE(drm_drv_uses_atomic_modeset(dev));
+
 		return false;
 	}
 
@@ -1338,6 +1340,10 @@ void drm_crtc_vblank_off(struct drm_crtc *crtc)
 		send_vblank_event(dev, e, seq, &now);
 	}
 	spin_unlock_irqrestore(&dev->event_lock, irqflags);
+
+	/* Will be reset by the modeset helpers when re-enabling the crtc by
+	 * calling drm_calc_timestamping_constants(). */
+	vblank->hwmode.crtc_clock = 0;
 }
 EXPORT_SYMBOL(drm_crtc_vblank_off);