drm/amd/display: Clear FEC_READY shadow register if DPCD write fails

[why]
As a fail-safe, in case 'set FEC_READY' DPCD write fails, a HW shadow
register should be cleared and the internal FEC stat should be set to
'not ready'. This is to make sure HW settings will be consistent with
FEC_READY state on the RX.
Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Joshua Aberback <Joshua.Aberback@amd.com>
Acked-by: Chris Park <Chris.Park@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -3174,6 +3174,8 @@ void 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->link_enc->funcs->fec_set_ready(link->link_enc, false);
+				link->fec_state = dc_link_fec_not_ready;
 				dm_error("dpcd write failed to set fec_ready");
 			}
 		} else if (link->fec_state == dc_link_fec_ready) {