gallium/xlib: Fix xmesa drawable creation

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.

Fixes: db8be355 (gallium/xlib: Remove drawable caching from the MakeCurrent path)
Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1196
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>