Use di_info_get_serial to fetch the serial number from EDID

This will also look at the "product serial string" in addition
to the actual serial number. The former is what we use elsewhere
in Plasma like the kscreen kcm.
BUG:485015
FIXED-IN:6.1

src/utils/edid.cpp

@@ -145,7 +145,8 @@ Edid::Edid(const void *data, uint32_t size)
     m_eisaId = parseEisaId(bytes);
     UniqueCPtr<char> monitorName{di_info_get_model(info)};
     m_monitorName = QByteArray(monitorName.get());
-    m_serialNumber = QByteArray::number(productInfo->serial);
+    UniqueCPtr<char> serial{di_info_get_serial(info)};
+    m_serialNumber = QByteArray(serial.get());
     m_vendor = parseVendor(bytes);
     QCryptographicHash hash(QCryptographicHash::Md5);
     hash.addData(m_raw);