https://bugs.freedesktop.org/show_bug.cgi?id=105433
--- Comment #7 from Thomas R. freedesktop-bugs@digital-trauma.de --- I've got news! With 4.19, the first modeset will still disable the second monitor, including when going into X (without mode change), but after changing the resolution to something else and back again, it now comes up reliably. This is much better than before. Also, I get this in dmesg
[drm] dce110_link_encoder_construct: Failed to get encoder_cap_info from VBIOS with error code 4!
on startup.
I don't know if this helps, and I'd be still willing to help debug given instructions.