u_vbuf: don't try to delete NULL driver CSO

Iago Toral requested to merge itoral/mesa:u_vbuf_delete_vertex_elements into master

Since 18a8c3f7 we don't create a driver CSO if there are any incompatible elements, so only ask backends to delete it if it exists.

Fixes multiple CTS crashes in V3D.

Fixes: 18a8c3f7 ("u_vbuf: Only create driver CSO if no incompatible elements")

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