drm: Add name for DRM_DP_DUAL_MODE_LSPCON

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: 056996b95686 ("drm: Helper for lspcon in drm_dp_dual_mode")
Cc: Shashank Sharma <[e-mail]>
Cc: Rodrigo Vivi <[e-mail]>
Cc: Dave Airlie <[e-mail]>
Cc: Ville Syrjälä <[e-mail]>
Cc: Jani Nikula <[e-mail]>
Cc: Daniel Vetter <[e-mail]>
Cc: Jani Nikula <[e-mail]>
Cc: Sean Paul <[e-mail]>
Cc: David Airlie <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Reviewed-by: Shashank Sharma <[e-mail]>
Signed-off-by: Ander Conselvan de Oliveira <[e-mail]>
Signed-off-by: Sean Paul <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_dp_dual_mode_helper.c

···
 		return "type 2 DVI";
 	case DRM_DP_DUAL_MODE_TYPE2_HDMI:
 		return "type 2 HDMI";
+	case DRM_DP_DUAL_MODE_LSPCON:
+		return "lspcon";
 	default:
 		WARN_ON(type != DRM_DP_DUAL_MODE_UNKNOWN);
 		return "unknown";