drm/amd/display: Constify dcn21_res_pool_funcs

The only usage of dcn21_res_pool_funcs is to assign its address to a
const pointer. Make it const to allow the compiler to put it in
read-only memory.
Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/dcn21/dcn21_resource.c

@@ -1754,7 +1754,7 @@ enum dc_status dcn21_patch_unknown_plane_state(struct dc_plane_state *plane_stat
 	return result;
 }
 
-static struct resource_funcs dcn21_res_pool_funcs = {
+static const struct resource_funcs dcn21_res_pool_funcs = {
 	.destroy = dcn21_destroy_resource_pool,
 	.link_enc_create = dcn21_link_encoder_create,
 	.panel_cntl_create = dcn21_panel_cntl_create,