drm/amd/display: Check link_res->hpo_dp_link_enc before using it

[WHAT & HOW]
Functions dp_enable_link_phy and dp_disable_link_phy can pass link_res
without initializing hpo_dp_link_enc and it is necessary to check for
null before dereferencing.

This fixes 1 FORWARD_NULL issue reported by Coverity.

Fixes: 0beca868 ("drm/amd/display: Check link_res->hpo_dp_link_enc before using it")
Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/link/hwss/link_hwss_hpo_dp.c

@@ -110,6 +110,11 @@ void enable_hpo_dp_link_output(struct dc_link *link,
 		enum clock_source_id clock_source,
 		const struct dc_link_settings *link_settings)
 {
+	if (!link_res->hpo_dp_link_enc) {
+		DC_LOG_ERROR("%s: invalid hpo_dp_link_enc\n", __func__);
+		return;
+	}
+
 	if (link->dc->res_pool->dccg->funcs->set_symclk32_le_root_clock_gating)
 		link->dc->res_pool->dccg->funcs->set_symclk32_le_root_clock_gating(
 				link->dc->res_pool->dccg,