gallium: fix memory corruption in u_gen_mipmap.c

gallium: fix memory corruption in u_gen_mipmap.c

Remove the old/initial vbuf allocation in util_create_gen_mipmap().
We were allocating a small vbuf at this point so get_next_slot() didn't have
as large of buffer as it expected.  So all but the first set_vertex_data()
was writing out of bounds.

Also added some comments.