drm/i915/lnl+/tc: Fix max lane count HW readout

On LNL+ for a disconnected sink the pin assignment value gets cleared by
the HW/FW as soon as the sink gets disconnected, even if the PHY
ownership got acquired already by the BIOS/driver (and hence the PHY
itself is still connected and used by the display). During HW readout
this can result in detecting the PHY's max lane count as 0 - matching
the above cleared aka NONE pin assignment HW state. For a connected PHY
the driver in general (outside of intel_tc.c) expects the max lane count
value to be valid for the video mode enabled on the corresponding output
(1, 2 or 4). Ensure this by setting the max lane count to 4 in this
case. Note, that it doesn't matter if this lane count happened to be
more than the max lane count with which the PHY got connected and
enabled, since the only thing the driver can do with such an output -
where the DP-alt sink is disconnected - is to disable the output.

v2: Rebased on change reading out the pin configuration only if the PHY
    is connected.

Cc: stable@vger.kernel.org # v6.8+
Reported-by: Charlton Lin <charlton.lin@intel.com>
Tested-by: Khaled Almahallawy <khaled.almahallawy@intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://lore.kernel.org/r/20250811080152.906216-4-imre.deak@intel.com

Author: ideak

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

@@ -23,6 +23,7 @@
 #include "intel_modeset_lock.h"
 #include "intel_tc.h"
 
+#define DP_PIN_ASSIGNMENT_NONE	0x0
 #define DP_PIN_ASSIGNMENT_C	0x3
 #define DP_PIN_ASSIGNMENT_D	0x4
 #define DP_PIN_ASSIGNMENT_E	0x5
@@ -308,6 +309,8 @@ static int lnl_tc_port_get_max_lane_count(struct intel_digital_port *dig_port)
 		REG_FIELD_GET(TCSS_DDI_STATUS_PIN_ASSIGNMENT_MASK, val);
 
 	switch (pin_assignment) {
+	case DP_PIN_ASSIGNMENT_NONE:
+		return 0;
 	default:
 		MISSING_CASE(pin_assignment);
 		fallthrough;
@@ -1159,6 +1162,12 @@ static void xelpdp_tc_phy_get_hw_state(struct intel_tc_port *tc)
 		tc->lock_wakeref = tc_cold_block(tc);
 
 		read_pin_configuration(tc);
+		/*
+		 * Set a valid lane count value for a DP-alt sink which got
+		 * disconnected. The driver can only disable the output on this PHY.
+		 */
+		if (tc->max_lane_count == 0)
+			tc->max_lane_count = 4;
 	}
 
 	drm_WARN_ON(display->drm,