xwayland: Detect gbm_bo_get_fd_for_plane at runtime

`gbm_bo_get_fd_for_plane` was introduced in Mesa 21 but some
proprietary GBM implementations (Xilinx) still haven't been updated
to support it:
```
/usr/bin/Xwayland: symbol lookup error: /usr/bin/Xwayland: undefined symbol: gbm_bo_get_fd_for_plane
```

Since distros would like to build a single Xwayland binary for all
OEMs of the same architecture, we now make the function always
available.

If a real implementation of `gbm_bo_get_fd_for_plane` exists at runtime
then it will be used, otherwise fall back to `gbm_bo_get_fd` or fail.
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