drm/atomic-helpers: Update vblank timestamping constants

The atomic helpers don't call drm_calc_timestamping_constants, which
is a regression compared to the crtc helpers. Fix this.

Noticed while reviewing i915 atomic patches from Maarten.

v2: Also check state->enable to avoid a warning in dmesg. Reported by
Maarten.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_atomic_helper.c

@@ -657,6 +657,10 @@ set_routing_links(struct drm_device *dev, struct drm_atomic_state *old_state)
 		crtc->enabled = crtc->state->enable;
 		crtc->x = crtc->primary->state->src_x >> 16;
 		crtc->y = crtc->primary->state->src_y >> 16;
+
+		if (crtc->state->enable)
+			drm_calc_timestamping_constants(crtc,
+							&crtc->state->adjusted_mode);
 	}
 }