https://bugs.freedesktop.org/show_bug.cgi?id=101499
--- Comment #10 from Carlo Caione carlo@caione.org ---
I think all of these are triggered by VRAM being too small to fit the scanout buffers covering the laptop panel + external monitor.
Probably I'm missing something, but when the HDMI is connected everything works fine, with the scanout buffer correctly displayed on the laptop panel + external monitor. The problem starts when we _disconnect_ the HDMI cable.
Also if it was a problem with VRAM being too small, why toggling the DPMS makes the laptop panel working fine again?