I managed to get hardware transcoding working!
First edit syslinux.cfg on the Unraid USB drive: nano /boot/syslinux/syslinux.cfg
Add the follow where I have shown below “append i915.enable_fbc=1 i915.enable_guc=2” and save your changes.
label Unraid OS
menu default
kernel /bzimage
append i915.enable_fbc=1 i915.enable_guc=2
append initrd=/bzroot
As the Unraid file system is loaded in to ram we want to create and edit an i915.conf in /boot/config/modprobe.d/ - this will persist though reboots.
nano /boot/config/modprobe.d/i915.conf
Add the following to i915.conf and save: options i915 enable_fbc=1 enable_guc=2
Reboot and the i915.conf will be present in /etc/modprobe.d/i915.conf
Add /dev/dri as a device to the Plex Docker if not already done.
Hardware transcoding will now work 
I had actually tried this before, however I’d edited /boot/syslinux/syslinux.cfg and /etc/modprobe.d/i915.conf then rebooted, so my /etc/modprobe.d/i915.conf config was lost due to the Unraid filesystem been loaded completely in ram, which I did not realise at the time.