Yes, when I installed the AMD Mobility Radeon driver 14.4 the GPU works fine again.
I had some trouble with my external monitor, because the Windows screen did not fill the whole monitor screen.
But in the AMD Catatyst Control Center I found out that I could adjust that with Underscan and Overscan.
Thank you for your help.
Don't forget to uncheck in AMD Catalyst Control Center, Software Update, the automated update checks.
Otherwise you end up with a newer version of the driver without knowing (I had an automated update to driver 15.4 without any notice)
I hope that Windows 10 not automatically overwrites driver 14.4
I got back to AMD Mobility Radeon driver 15.7 because this driver works better with unchecked GPU box than driver 14.4 with checked GPU box (Windows 10 and Lightroom 6.1.1).