I have same issue using either LIghtDM or GDM with HP zBook 15u.  I experience the problem as being problematic when the laptop is either suspended from an undocked state using the internal display, then resumed while connected to the dock with dual external displays.  The resume does not seem to detect the docked displays and activate them automatically, leaving me to open the lid on the dock and manually configure them.  I have also checked the "configure new displays when connected" box, but the dialog for this appears on the internal screen, while the external screens remain in powersave mode.

My previous laptop which was an Elitebook 8400 series running 12.04 did not have this issue with similar configuration.

# lspci
00:00.0 Host bridge: Intel Corporation Broadwell-U Host Bridge -OPI (rev 09)
00:02.0 VGA compatible controller: Intel Corporation Broadwell-U Integrated Graphics (rev 09)
00:03.0 Audio device: Intel Corporation Broadwell-U Audio Controller (rev 09)
00:14.0 USB controller: Intel Corporation Wildcat Point-LP USB xHCI Controller (rev 03)
00:16.0 Communication controller: Intel Corporation Wildcat Point-LP MEI Controller #1 (rev 03)
00:19.0 Ethernet controller: Intel Corporation Ethernet Connection (3) I218-LM (rev 03)
00:1b.0 Audio device: Intel Corporation Wildcat Point-LP High Definition Audio Controller (rev 03)
00:1c.0 PCI bridge: Intel Corporation Wildcat Point-LP PCI Express Root Port #1 (rev e3)
00:1c.1 PCI bridge: Intel Corporation Wildcat Point-LP PCI Express Root Port #2 (rev e3)
00:1c.3 PCI bridge: Intel Corporation Wildcat Point-LP PCI Express Root Port #4 (rev e3)
00:1c.4 PCI bridge: Intel Corporation Wildcat Point-LP PCI Express Root Port #5 (rev e3)
00:1c.5 PCI bridge: Intel Corporation Wildcat Point-LP PCI Express Root Port #6 (rev e3)
00:1d.0 USB controller: Intel Corporation Wildcat Point-LP USB EHCI Controller (rev 03)
00:1f.0 ISA bridge: Intel Corporation Wildcat Point-LP LPC Controller (rev 03)
00:1f.3 SMBus: Intel Corporation Wildcat Point-LP SMBus Controller (rev 03)
02:00.0 Unassigned class [ff00]: Realtek Semiconductor Co., Ltd. RTS5227 PCI Express Card Reader (rev 01)
03:00.0 Network controller: Intel Corporation Wireless 3160 (rev 83)
04:00.0 VGA compatible controller: Advanced Micro Devices, Inc. [AMD/ATI] Opal XT [Radeon R7 M265] (rev ff)
05:00.0 SATA controller: Marvell Technology Group Ltd. 88SS9183 PCIe SSD Controller (rev 14)

Running the xorg driver, tried the AMD/ATI driver but it seemed to be significantly less stable with graphical fractures in windows, xorg crashes and more problems with suspend/resume operations.

# glxinfo |grep -i vendor
server glx vendor string: SGI
client glx vendor string: Mesa Project and SGI
OpenGL vendor string: Intel Open Source Technology Center


# lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 15.04
Release:	15.04
Codename:	vivid

# uname -a
Linux xyz 3.19.0-22-generic #22-Ubuntu SMP Tue Jun 16 17:15:15 UTC 2015 x86_64 x86_64 x86_64 GNU/Linux

# dpkg-query -l|egrep "(g|light)dm "
ii  gdm                                         3.14.1-0ubuntu3                         amd64        Next generation GNOME Display Manager
ii  lightdm                                     1.14.2-0ubuntu1                         amd64        Display Manager