svga: avoid emitting redundant SetShaderResources and SetVertexBuffers

svga: avoid emitting redundant SetShaderResources and SetVertexBuffers

Minor performance improvement in avoiding binding the same shader resource
or the same vertex buffer for the same slot.

Tested with MTT glretrace.

v2: Per Brian's suggestion, add a helper function to do vertex buffer
    comparision.
v3: Change the helper function to vertex_buffers_equal().

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