xwayland: Use the pixmap release callback with EGLStream

Commit 77658741 - "xwayland: Add buffer release callback" added an API
to deal with Wayland buffer release callbacks.

The EGLstream implementation has its own wl_buffer callback, move that
to the buffer release API instead so we don't have to deal with Wayland
buffers directly and match the other Xwayland pixmap backend
implementations.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
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