Comment # 16 on bug 1193397 from Takashi Iwai

Try to boot with monitor off, run X.  Then before turning on the monitor,
enable the debug option of drm, run the following as root:
   echo 0x1e > /sys/module/drm/parameters/debug

Then turn on the monitor, confirm the problem ('no signal').  Save the dmesg
output at this stage.

Then move to VT1, restore the monitor out, and save the dmesg output again.
After that, clear the drm debug option
    echo 0 > /sys/module/drm/parameters/debug

Upload the saved two dmesg outputs to Bugzilla.

BTW, when you boot with another desktop environment (such as icewm), does the
problem persist?  I mean not to log out and switch from KDE, but login directly
to icewm session (which should be available as fallback) at boot.