All the DRI driver did was call the createDrawable callback immediately
upon entry to DRIscreen::createNewDrawable to get the drm_drawable_t.
We can just call that before calling into the DRI driver and pass the
returned drm_drawable_t as an argument to the DRI entry point.
Likewise for destroyDrawable.
Also, DRIdrawablePrivate::draw isn't used anywhere, and since the
driver no longer needs the XID of the drawable we can now drop that.
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