drm/i915/psr: Fall back to max. synchronization latency if DPCD read fails

Noticed that we assume the best case of 0 latency when the DPCD read
fails, reasonable pessimism is safer.

eDP spec does say that if latency is greater than 8, the panel
supplier needs to provide it. I didn't see anything specific in the VBT
for this, so let's go with 8 frames as a fallback.

Cc: Jose Roberto de Souza <jose.souza@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180511195145.3829-5-dhinakaran.pandiyan@intel.com

drivers/gpu/drm/i915/intel_psr.c

@@ -219,13 +219,13 @@ static bool intel_dp_get_alpm_status(struct intel_dp *intel_dp)
 
 static u8 intel_dp_get_sink_sync_latency(struct intel_dp *intel_dp)
 {
-	u8 val = 0;
+	u8 val = 8; /* assume the worst if we can't read the value */
 
 	if (drm_dp_dpcd_readb(&intel_dp->aux,
 			      DP_SYNCHRONIZATION_LATENCY_IN_SINK, &val) == 1)
 		val &= DP_MAX_RESYNC_FRAME_COUNT_MASK;
 	else
-		DRM_ERROR("Unable to get sink synchronization latency\n");
+		DRM_DEBUG_KMS("Unable to get sink synchronization latency, assuming 8 frames\n");
 	return val;
 }