drm/amdgpu: check if hubbub is NULL in debugfs/amdgpu_dm_capabilities

HUBBUB structure is not initialized on DCE hardware, so check if it is NULL
to avoid null dereference while accessing amdgpu_dm_capabilities file in
debugfs.

Signed-off-by: Peter Shkenev <mustela@erminea.space>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

Author: Peter Shkenev

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_debugfs.c

@@ -3988,7 +3988,7 @@ static int capabilities_show(struct seq_file *m, void *unused)
 
 	struct hubbub *hubbub = dc->res_pool->hubbub;
 
-	if (hubbub->funcs->get_mall_en)
+	if (hubbub && hubbub->funcs->get_mall_en)
 		hubbub->funcs->get_mall_en(hubbub, &mall_in_use);
 
 	if (dc->cap_funcs.get_subvp_en)