drm/i915/xe3p_lpd: Always apply WaWmMemoryReadLatency

When reading memory latencies for watermark calculations, previous
display releases instructed to apply an adjustment of adding a certain
value (e.g. 6us) to all levels when the level 0's memory latency read
from hardware was zero.

For Xe3p_LPD, the instruction is to always use 6us for level 0 and to
add that value to the other levels.  Add the necessary code in
sanitize_wm_latency() so that WaWmMemoryReadLatency is always applied
for Xe3p_LPD and beyond.

v2:
  - Rebased after addition of prep patch "drm/i915/wm: Reorder
    adjust_wm_latency() for Xe3_LPD" (dropped in v3).
v3:
  - Back to the simpler approach of doing the 'wm[0] = 0' step without
    modifying the rest of the code, and that inside
    sanitize_wm_latency(). (Matt Roper, Ville)

Bspec: 68986, 69126
Cc: Matt Atwood <matthew.s.atwood@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patch.msgid.link/20251103-xe3p_lpd-basic-enabling-v3-20-00e87b510ae7@intel.com
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>

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

@@ -3184,6 +3184,13 @@ static void sanitize_wm_latency(struct intel_display *display)
 	u16 *wm = display->wm.skl_latency;
 	int level, num_levels = display->wm.num_levels;
 
+	/*
+	 * Xe3p and beyond should ignore level 0's reported latency and
+	 * always apply WaWmMemoryReadLatency logic.
+	 */
+	if (DISPLAY_VER(display) >= 35)
+		wm[0] = 0;
+
 	/*
 	 * If a level n (n > 1) has a 0us latency, all levels m (m >= n)
 	 * need to be disabled. We make sure to sanitize the values out