drm/plane_helper: don't disable plane in destroy function

By the time drm_mode_config_cleanup calls this all the hw state should
be cleaned up already - we even have a WARN right before calling
plane->destroy callbacks asserting that all framebuffers are gone.

So trying to disable things harder is a bit a bug. Caught by Thierry
since it resulted in some mode_config.mutex locking backtraces.

Cc: Thierry Reding <treding@nvidia.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: NThierry Reding <treding@nvidia.com>
Tested-by: NThierry Reding <treding@nvidia.com>
Reviewed-by: NMatt Roper <matthew.d.roper@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_plane_helper.c

@@ -234,7 +234,6 @@ EXPORT_SYMBOL(drm_primary_helper_disable);
  */
 void drm_primary_helper_destroy(struct drm_plane *plane)
 {
-	plane->funcs->disable_plane(plane);
 	drm_plane_cleanup(plane);
 	kfree(plane);
 }