drm/amd/display: Improve logic for is_lttpr_present

[WHY]
DP specifies that an LTTPR device is only present if PHY_REPEATER_CNT is
0x80, 0x40, 0x20, 0x10, 0x08, 0x04, 0x02, or 0x01.

All other values should be considered no LTTPRs present.

[HOW]
Function dp_convert_to_count already does this check. Use it to determine
if PHY_REPEATER_CNT is a valid LTTPR count.
Signed-off-by: Wesley Chalmers <Wesley.Chalmers@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@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

@@ -3699,8 +3699,7 @@ bool dp_retrieve_lttpr_cap(struct dc_link *link)
 								DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV];
 
 		/* Attempt to train in LTTPR transparent mode if repeater count exceeds 8. */
-		is_lttpr_present = (link->dpcd_caps.lttpr_caps.phy_repeater_cnt > 0 &&
-				link->dpcd_caps.lttpr_caps.phy_repeater_cnt < 0xff &&
+		is_lttpr_present = (dp_convert_to_count(link->dpcd_caps.lttpr_caps.phy_repeater_cnt) != 0 &&
 				link->dpcd_caps.lttpr_caps.max_lane_count > 0 &&
 				link->dpcd_caps.lttpr_caps.max_lane_count <= 4 &&
 				link->dpcd_caps.lttpr_caps.revision.raw >= 0x14);