drm/i915: Allow 27 bytes child_dev for VBT <109

My 85x has VBT version 108 which has a child dev size of 27 bytes.
Let's allow that without printing an error.

We still want to reject the actual parsin since for that we need
the child device size to be at least 33 bytes. So we should still
check for that, but let's make it print a debug message only instead
of an error.

While at it, toss in a BUILD_BUG_ON() to verify our struct
old_child_dev_config is in fact 33 bytes.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1450110229-30450-8-git-send-email-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_bios.c

@@ -1157,7 +1157,10 @@ parse_device_mapping(struct drm_i915_private *dev_priv,
 		DRM_DEBUG_KMS("No general definition block is found, no devices defined.\n");
 		return;
 	}
-	if (bdb->version < 195) {
+	if (bdb->version < 109) {
+		expected_size = 27;
+	} else if (bdb->version < 195) {
+		BUILD_BUG_ON(sizeof(struct old_child_dev_config) != 33);
 		expected_size = sizeof(struct old_child_dev_config);
 	} else if (bdb->version == 195) {
 		expected_size = 37;
@@ -1170,18 +1173,18 @@ parse_device_mapping(struct drm_i915_private *dev_priv,
 				 bdb->version, expected_size);
 	}
 
-	/* The legacy sized child device config is the minimum we need. */
-	if (p_defs->child_dev_size < sizeof(struct old_child_dev_config)) {
-		DRM_ERROR("Child device config size %u is too small.\n",
-			  p_defs->child_dev_size);
-		return;
-	}
-
 	/* Flag an error for unexpected size, but continue anyway. */
 	if (p_defs->child_dev_size != expected_size)
 		DRM_ERROR("Unexpected child device config size %u (expected %u for VBT version %u)\n",
 			  p_defs->child_dev_size, expected_size, bdb->version);
 
+	/* The legacy sized child device config is the minimum we need. */
+	if (p_defs->child_dev_size < sizeof(struct old_child_dev_config)) {
+		DRM_DEBUG_KMS("Child device config size %u is too small.\n",
+			      p_defs->child_dev_size);
+		return;
+	}
+
 	/* get the block size of general definitions */
 	block_size = get_blocksize(p_defs);
 	/* get the number of child device */