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 2 FORWARD_NULL issues reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alex Hung <alex.hung@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/link/hwss/link_hwss_hpo_dp.c

@@ -28,6 +28,8 @@
 #include "dccg.h"
 #include "clk_mgr.h"
 
+#define DC_LOGGER link->ctx->logger
+
 void set_hpo_dp_throttled_vcp_size(struct pipe_ctx *pipe_ctx,
 		struct fixed31_32 throttled_vcp_size)
 {
@@ -124,6 +126,11 @@ void disable_hpo_dp_link_output(struct dc_link *link,
 		const struct link_resource *link_res,
 		enum signal_type signal)
 {
+	if (!link_res->hpo_dp_link_enc) {
+		DC_LOG_ERROR("%s: invalid hpo_dp_link_enc\n", __func__);
+		return;
+	}
+
 		link_res->hpo_dp_link_enc->funcs->link_disable(link_res->hpo_dp_link_enc);
 		link_res->hpo_dp_link_enc->funcs->disable_link_phy(
 				link_res->hpo_dp_link_enc, signal);