drm-buffer: Keep track of handle as well

This handle is used by the legacy KMS API; lets avoid having to have GBM
specific code where this is done by letting the MetaDrmBuffer API, that
already has this information, expose it.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2275>

src/backends/native/meta-drm-buffer-dumb.c

@@ -248,6 +248,7 @@ init_dumb_buffer (MetaDrmBufferDumb  *buffer_dumb,
     .width = width,
     .height = height,
     .format = format,
+    .handle = create_arg.handle,
     .handles = { create_arg.handle },
     .strides = { create_arg.pitch },
   };