There is a bit of confusion in the GBM platform initialisation

static gboolean
init_gbm (MetaRendererNative *renderer_native,
          GError            **error)

in src/backends/native/meta-renderer-native.c

The code checks for the extension:
EGL_MESA_platform_gbm

and if that is missing, you get the following error message:
"Missing extension for GBM renderer: EGL_KHR_platform_gbm"

I think the check should be for EGL_MESA_platform_gbm || EGL_KHR_platform_gbm as there are non-MESA platforms that use EGL_KHR_platform_gbm. One example of this is Mali GPU, which has a driver that supports Wayland and exports EGL_KHR_platform_gbm, but because mutter checks for EGL_MESA_platform_gbm, it fails initialising gbm.