gallium/xlib: Fix xmesa drawable creation
The first time you call glXMakeCurrent, current != ctx. As a result we would never look up whether the drawable already had an XMesaDrawable, and would instead always create one. Then XMesaBufferList would have two different buffers for the same XID, and you'd be reading and drawing to different places, and that's not what you want at all.
Instead just always look up the drawable.