xwayland: search for a render node to use

wl_drm's protocol "device" event provides the path to the DRM device,
which may not be a render node, thus causing Xwayland to fall back to
DRM authentication which may fail if the user has switched to another
VT while Xwayland is starting.

Search for a render node corresponding to the given DRM device and try
to use it instead, as render nodes do not need DRM authentication and
Xwayland can make use of them if it can find one.

Closes: https://bugs.freedesktop.org/108038
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
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