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