drm/amd/display: fix dp kvm can't light up

[why]
The DP KVM failed to light up, since the lttpr_mode is not reset to
default value when failed to read LTTPR capabilities, and the
variable max_link_rate retains a initial value zero, this cause variable
link_rate be assigned to an error value zero, consquently pixel_clock
get wrong value zero, and kvm can't light up.

[how]
Reset lttpr_mode to default value when failed to read LTTPR
capabilities, so that the link_rate fallbacks to the minimum rate that
supported by link encoder and sink.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Tianci.Yin <tianci.yin@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -5110,6 +5110,7 @@ bool dp_retrieve_lttpr_cap(struct dc_link *link)
 				sizeof(lttpr_dpcd_data));
 		if (status != DC_OK) {
 			DC_LOG_DP2("%s: Read LTTPR caps data failed.\n", __func__);
+			link->lttpr_mode = LTTPR_MODE_NON_LTTPR;
 			return false;
 		}