drm/i915: Do not return unknown status when load detection is tested.

This fixes the IGT test, which interprets unknown status as failed to
acquire load detect pipe.

Cc: Gabriel Feceoru <gabriel.feceoru@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456848241-6431-1-git-send-email-maarten.lankhorst@linux.intel.com


Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>