libexternal: enable HPD if target supports HDMI

enable HPD only on targets that has valid HDMI fb device.

Change-Id: Ia43d9138167b6ab3a887f7663a05831a2fcc09ea
CRs-fixed: 506363

libexternal/external.cpp

@@ -670,6 +670,7 @@ int ExternalDisplay::getExtFbNum(int &fbNum) {
 bool ExternalDisplay::writeHPDOption(int userOption) const
 {
     bool ret = true;
+    if(mHdmiFbNum != -1) {
         char sysFsHPDFilePath[MAX_SYSFS_FILE_PATH];
         snprintf(sysFsHPDFilePath ,sizeof(sysFsHPDFilePath),
                  "/sys/devices/virtual/graphics/fb%d/hpd", mHdmiFbNum);
@@ -691,6 +692,7 @@ bool ExternalDisplay::writeHPDOption(int userOption) const
             }
             close(hdmiHPDFile);
         }
+    }
     return ret;
 }