graphics-drivers/intel: add IOMMU troubleshooting

src/config/graphical-session/graphics-drivers/intel.md

@@ -24,3 +24,14 @@ environment variable `LIBVA_DRIVER_NAME`:
 |----------------------|-------------------|--------------------------|
 | `libva-intel-driver` | up to Coffee Lake | `LIBVA_DRIVER_NAME=i965` |
 | `intel-media-driver` | from Broadwell    | `LIBVA_DRIVER_NAME=iHD`  |
+
+## Troubleshooting
+
+The kernels packaged by Void are configured with
+`CONFIG_INTEL_IOMMU_DEFAULT_ON=y`, which can lead to issues with their graphics
+drivers, as reported by the [kernel
+documentation](https://www.kernel.org/doc/html/latest/x86/intel-iommu.html#graphics-problems).
+To fix this, it is necessary to disable IOMMU for the integrated GPU. This can
+be done by adding `intel_iommu=igfx_off` to your [kernel
+cmdline](../../kernel.md#cmdline). If your issues are fixed by this kernel
+option, you should file a bug reporting the problem to kernel developers.