drm/amd/display: Fix null pointer dereference for encoders

[Why]
Links which are dynamically assigned link encoders have their link
encoder set to NULL.

[How]
Check that a pointer to a link_encoder object is non-NULL before using
it.

Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Reviewed-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/dc/core/dc_link_dp.c

@@ -5705,7 +5705,7 @@ enum dc_status dp_set_fec_ready(struct dc_link *link, bool ready)
 				link_enc->funcs->fec_set_ready(link_enc, true);
 				link->fec_state = dc_link_fec_ready;
 			} else {
-				link_enc->funcs->fec_set_ready(link->link_enc, false);
+				link_enc->funcs->fec_set_ready(link_enc, false);
 				link->fec_state = dc_link_fec_not_ready;
 				dm_error("dpcd write failed to set fec_ready");
 			}

drivers/gpu/drm/amd/display/dc/dcn10/dcn10_hw_sequencer.c

@@ -1587,7 +1587,7 @@ void dcn10_power_down_on_boot(struct dc *dc)
 		for (i = 0; i < dc->link_count; i++) {
 			struct dc_link *link = dc->links[i];
 
-			if (link->link_enc->funcs->is_dig_enabled &&
+			if (link->link_enc && link->link_enc->funcs->is_dig_enabled &&
 					link->link_enc->funcs->is_dig_enabled(link->link_enc) &&
 					dc->hwss.power_down) {
 				dc->hwss.power_down(dc);