Reviewing the driver list for the system at the following site, it doesn't look like this unit supports Windows 8.
http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?cc=us&dlc=en&docname=c02974941&lc=en&product=5167397&query=dv6%206155tx&tool=&
It looks like the system uses switchable graphics. See the following thread for a process that may get this working for you.
http://h30434.www3.hp.com/t5/Notebook-Display-and-Video-e-g-Windows-8/I-upgraded-to-Windows-8-How-do-I-get-switchable-graphics-to/m-p/2213275/highlight/true#M59120
http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?cc=us&dlc=en&docname=c02974941&lc=en&product=5167397&query=dv6%206155tx&tool=&
It looks like the system uses switchable graphics. See the following thread for a process that may get this working for you.
http://h30434.www3.hp.com/t5/Notebook-Display-and-Video-e-g-Windows-8/I-upgraded-to-Windows-8-How-do-I-get-switchable-graphics-to/m-p/2213275/highlight/true#M59120