xwayland/glamor: make wl_drm optional

With zwp_linux_dmabuf_feedback_v1, wl_drm becomes optional.  There are
also compositors (i.e., Chrome EXO) that support only
zwp_linux_dmabuf_v1.
9 jobs for optional-wl-drm in 1 minute and 47 seconds (queued for 3 seconds)
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