Mac Pro 2013 Blank Displays And Can't Hard Reset
I think this may be one of the few times that recommending reset of the SMC & NVRAM might be in order.
NVRAM stores display config, amongst other things - see The Eclectic Light Company - What is stored in NVRAM? (generally a knowledgeable resource)
SMC stores external video source in some Macs. I'm less certain this is going to be the useful step, but the same resource, The Eclectic Light Company mentions that treating them as a pair can be useful. SMC first, then NVRAM.
Persistence: EFI, SMC and NVRAM