Arch Linux is my daily driver. And still I tend to fiddle around and break stuff, not on purpose, but that sometimes just happens. So it is nice to have a backup plan for general purpose errors, which prohibit your system from booting.
Plugin a live USB (I use Antergos) and run the live system
In a terminal in the live system check the available disks and choose the one with the encrypted partition via following command
Next decrypt the LUKS partition (here /dev/sda2) so you can mount it in the next step
sudo cryptsetup luksOpen /dev/sda2 luks-partition
Intel provides a nice tool which comes with this ability out of the box, called intel-gpu-tools. It is actually meant for debugging the Intel graphics driver, but fulfills our needs.
Using Arch Linux, intel-gpu-tools can be simply installed via sudo pacman -S intel-gpu-tools or by using a pacman wrapper of your choice, like yaourt.
The tool comes with several commands, but intel_gpu_top is the one we are looking for. Via sudo intel_gpu_top you can get a top like overview of the current gpu usage.
If you are using another Linux based operating system, you can download the source here and also find packages for Debian, Fedora and Ubuntu via the corresponding package managers.