drm/i915: Zero the mode in intel_sanitize_crtc when force disabling.

There is a WARN_ON in drm_atomic_crtc_check for this when exposing the atomic property.
If the mode_blob still exists, but enable = false then all updates are rejected with -EINVAL.
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -15291,7 +15291,7 @@ static void intel_sanitize_crtc(struct intel_crtc *crtc)
 			      crtc->base.state->enable ? "enabled" : "disabled",
 			      crtc->active ? "enabled" : "disabled");
 
-		crtc->base.state->enable = crtc->active;
+		WARN_ON(drm_atomic_set_mode_for_crtc(crtc->base.state, NULL) < 0);
 		crtc->base.state->active = crtc->active;
 		crtc->base.enabled = crtc->active;