ilo: 3DSTATE_INDEX_BUFFER may be wrongly skipped

ilo: 3DSTATE_INDEX_BUFFER may be wrongly skipped

In finalize_index_buffer(), when the current index buffer was destroyed due to
u_upload_data(), it may happen that the new index buffer is at the same
address as the old one.  Comparing the pointers to the two buffers could fail
to work, and 3DSTATE_INDEX_BUFFER would be incorrectly skipped.

Holding a reference to the current index buffer before calling u_upload_data()
should fix the problem.