Handle DRM_DP_DUAL_MODE_LSPCON in drm_dp_get_dual_mode_type_name(),
otherwise a call to that function can theoretically trigger a WARN.

Fixes: 056996b9 ("drm: Helper for lspcon in drm_dp_dual_mode")
Cc: Shashank Sharma <shashank.sharma@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Dave Airlie <airlied@gmail.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Cc: intel-gfx@lists.freedesktop.org
Reviewed-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170222125928.30223-1-ander.conselvan.de.oliveira@intel.com