Pass EGL_PLATFORM=DRM to XWayland
XWayland needs to create an eglDisplay for platform DRM. Depending on
the mesa version eglInitialize expects that the passed native display
is a Wayland display and crashes. This prevents KWin to startup.
This patch sets EGL_PLATFORM for Xwayland. This is actually not KWin's
job, but if it otherwise crashes it's better than nothing.
As Xwayland only supports DRM, it's fine to use this.