drm/i915/dp: Reset cached LTTPR count if number of LTTPRs is unsupported

After detection the cached LTTPR count can be checked to determine if
LTTPRs in non-transparent mode were detected. Reset the cached LTTPR
count if the reported number of LTTPRs is invalid to ensure the above
checks work as expected.
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240708190029.271247-4-imre.deak@intel.com

drivers/gpu/drm/i915/display/intel_dp_link_training.c

@@ -174,7 +174,7 @@ static int intel_dp_init_lttpr_phys(struct intel_dp *intel_dp, const u8 dpcd[DP_
 	 * still taking into account any LTTPR common lane- rate/count limits.
 	 */
 	if (lttpr_count < 0)
-		return 0;
+		goto out_reset_lttpr_count;
 
 	if (!intel_dp_set_lttpr_transparent_mode(intel_dp, false)) {
 		lt_dbg(intel_dp, DP_PHY_DPRX,