Hi Tero,
If I remember well, HP has a weird way of routing its HDMI output that Premiere my not detect. At least it was the case with the DV 9000 series. If your monitor is set as extended (important), also make sure you restart Premiere after you set up the monitor.
Otherwise, it's likely an HP issue (like many others, sorry personal grudge :-)
Vince Becquiot
Kaptis Studios
San Francisco - Bay Area