drm/amd/display: Add null pointer check to is_dig_enabled func

[WHY]
Safeguarding as pointer may be null in diagnostic environment
Signed-off-by: Michael Strauss <michael.strauss@amd.com>
Reviewed-by: Sung Lee <Sung.Lee@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/clk_mgr/dcn301/vg_clk_mgr.c

@@ -75,7 +75,8 @@ int vg_get_active_display_cnt_wa(
 		const struct dc_link *link = dc->links[i];
 
 		/* abusing the fact that the dig and phy are coupled to see if the phy is enabled */
-		if (link->link_enc->funcs->is_dig_enabled(link->link_enc))
+		if (link->link_enc->funcs->is_dig_enabled &&
+				link->link_enc->funcs->is_dig_enabled(link->link_enc))
 			display_count++;
 	}