svga: fix vertex buffer references in the hw state

svga: fix vertex buffer references in the hw state

This patch fixes three issues with vertex buffer references:
(1) Instead of copy the vertex buffer resource handles to the hw state
    in the context structure, use pipe_resource_reference to properly
    reference the vertex buffer resources in the context.

(2) Make sure to unbind those unused vertex buffer resources.

(3) Force to rebind the vertex buffer resources at the first draw of each
    command buffer to make sure the vertex buffer resources are paged in.

Reviewed-by: Brian Paul <brianp@vmware.com>