[intel-gem] Add DRM_IOCTL_I915_GEM_SW_FINISH to flag CPU writes

[intel-gem] Add DRM_IOCTL_I915_GEM_SW_FINISH to flag CPU writes

When a software fallback has completed, usermode must notify the kernel so
that any scanout buffers can be synchronized. This ioctl should be called
whenever a fallback completes to flush CPU and chipset caches.