Have the same problem with a rtx 2070. My temporary solution is to put a cheap gpu into my pc(nvidia geforce 710) into the second pci port and cut of the power of the 2070 when using qubes. the problem is simple. The included nouveau driver does not support it. You could install the properitary one whats a security problem. my attemp to solve it, when I've got time, will be to compile the newest nouveau driver in a fedora 25 qubes and install it to dom0. For sure its necessary to ensure cube nit getting compromised in the process.
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