clients/simple-dmabuf-egl: Create the EGL display using the GBM platform

Since we are managing and rendering to buffers on our own with GBM,
create the EGL display using the GBM platform with the DRM render node,
instead of using the Wayland EGL platform.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>