target: apq8084: correct the command line buffer size check

Provide proper string length for HDMI command line check to
avoid wrong buffer length check

Change-Id: I34ad9f01debfa28687d7ede7aa47a73192a69569
diff --git a/target/apq8084/target_display.c b/target/apq8084/target_display.c
index 3aa2379..5b2d77a 100755
--- a/target/apq8084/target_display.c
+++ b/target/apq8084/target_display.c
@@ -371,7 +371,7 @@
 
 	if (!strcmp(panel_name, HDMI_PANEL_NAME)) {
 		if (buf_size < (prefix_string_len + LK_OVERRIDE_PANEL_LEN +
-				HDMI_CONTROLLER_STRING)) {
+				strlen(HDMI_CONTROLLER_STRING))) {
 			dprintf(CRITICAL, "command line argument is greater than buffer size\n");
 			return false;
 		}