For tracking this regression down I didn't build any drivers for the gpu and booted to text console.
Until and include linux-3.14.x, resume was fine. 3.15 - 4.2.7 does not work
After new boot
>lspci -v -s 07:00.0
gives me
07:00.0 3D controller: NVIDIA Corporation GK208M [GeForce GT 740M] (rev a1)
	Subsystem: Lenovo GK208M [GeForce GT 740M]
	Flags: bus master, fast devsel, latency 0
	Memory at b3000000 (32-bit, non-prefetchable) [size=16M]
	Memory at a0000000 (64-bit, prefetchable) [size=256M]
	Memory at b0000000 (64-bit, prefetchable) [size=32M]
	I/O ports at 4000 [disabled] [size=128]
	Expansion ROM at <ignored> [disabled]
	Capabilities: [60] Power Management version 3
	Capabilities: [68] MSI: Enable- Count=1/1 Maskable- 64bit+
	Capabilities: [78] Express Endpoint, MSI 00
	Capabilities: [100] Virtual Channel
	Capabilities: [128] Power Budgeting <?>
	Capabilities: [600] Vendor Specific Information: ID=0001 Rev=1 Len=024 <?>
	Capabilities: [900] #19

I can then load bbswitch and turn the gpu on and off as I want, works.
Enter resume using
>systemctl suspend
><press any key to resume>
With linux-3.14.x lspci, bbswitch work as before. From 3.15 on, lspci gives me:
07:00.0 3D controller: NVIDIA Corporation GK208M [GeForce GT 740M] (rev ff) (prog-if ff)
	!!! Unknown header type 7f

Loading bbswitch and trying to switch gives:
Device refused to change power state. Currently in D3

in 3.15 there were many changes regarding power management and faster resume. Noteable patches using async suspend/resume so I tried with that but no success.
Having CONFIG_PM_ADVANCED_DEBUG=y set,
/sys/devices/pci0000:00/0000:00:01.1/0000:07:00.0/power/async is set to enabled.
This is an oddity by itself as the default value should be disabled and there is no driver that could set that value. What sets this?
Long story short, neither setting this to disabled nor setting 
/sys/power/pm_async
to 0 changes anything, still doesn't work.
Out of ideas now where to look, please advise.
PCI-ID: 10de:1292
Regards.