drm/amd/display: Assume an LTTPR is always present on fixed_vs links

[WHY]
LTTPRs can in very rare instsances fail to increment DPCD LTTPR count.
This results in aux-i LTTPR requests to be sent to the wrong DPCD
address, which causes link training failure.

[HOW]
Override internal repeater count if fixed_vs flag is set for a given link

Reviewed-by: George Shen <George.Shen@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Michael Strauss <michael.strauss@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/core/dc_link_dp.c

@@ -5133,6 +5133,14 @@ bool dp_retrieve_lttpr_cap(struct dc_link *link)
 				lttpr_dpcd_data[DP_PHY_REPEATER_128B132B_RATES -
 								DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV];
 
+		/* If this chip cap is set, at least one retimer must exist in the chain
+		 * Override count to 1 if we receive a known bad count (0 or an invalid value) */
+		if (link->chip_caps & EXT_DISPLAY_PATH_CAPS__DP_FIXED_VS_EN &&
+				(dp_convert_to_count(link->dpcd_caps.lttpr_caps.phy_repeater_cnt) == 0)) {
+			ASSERT(0);
+			link->dpcd_caps.lttpr_caps.phy_repeater_cnt = 0x80;
+		}
+
 		/* Attempt to train in LTTPR transparent mode if repeater count exceeds 8. */
 		is_lttpr_present = (link->dpcd_caps.lttpr_caps.max_lane_count > 0 &&
 				link->dpcd_caps.lttpr_caps.max_lane_count <= 4 &&