Bug#797999: sddm fails to start whereas kdm works correctly

[Eric Valette <eric2.valette@orange.com>]

Package: sddm
Version: 0.11.0-4
Severity: important

If I do dpkg-reconfigure sddm, select it and reboot, I just get a black
screen, with X running, no cursor, no mouse nothing in/var/log/sddm.log

Note its a docked laptop, with external monitor used, lid closed and laptop
monitor automatically desactivated.

-- System Information:
Debian Release: stretch/sid
  APT prefers unstable
  APT policy: (500, 'unstable'), (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 4.1.6 (SMP w/4 CPU cores; PREEMPT)
Locale: LANG=en_US.UTF8, LC_CTYPE=en_US.UTF8 (charmap=UTF-8) (ignored: LC_ALL set to en_US.UTF8)
Shell: /bin/sh linked to /bin/bash
Init: systemd (via /run/systemd/system)

Versions of packages sddm depends on:
ii  adduser                         3.113+nmu3
ii  debconf [debconf-2.0]           1.5.57
ii  libc6                           2.21-0experimental1
ii  libgcc1                         1:5.2.1-16
ii  libpam0g                        1.1.8-3.1
ii  libqt5core5a                    5.4.2+dfsg-9
ii  libqt5dbus5                     5.4.2+dfsg-9
ii  libqt5gui5                      5.4.2+dfsg-9
ii  libqt5network5                  5.4.2+dfsg-9
ii  libqt5qml5                      5.4.2-6
ii  libqt5quick5                    5.4.2-6
ii  libstdc++6                      5.2.1-16
ii  libsystemd0                     225-1
ii  libxcb-xkb1                     1.10-3+b1
ii  libxcb1                         1.10-3+b1
ii  qml-module-qtquick2             5.4.2-6
ii  sddm-theme-breeze [sddm-theme]  4:5.3.2-4+b1

sddm recommends no packages.

Versions of packages sddm suggests:
pn  pam-kwallet5  <none>

-- debconf information:
  sddm/daemon_name: /usr/bin/sddm
* shared/default-x-display-manager: kdm