drm/i915: Restore planes after load detection

Actually turn the planes back on after were done with
the load detection.

Fixes: 20bdc112 ("drm/i915: Disable all planes for load detection, v2.")
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180322152313.6561-23-ville.syrjala@linux.intel.com