svga: fix draw elements with 8-bits indices
With commit 2122b902, u_index_translator can return U_TRANSLATE_MEMCPY for 8-bits indices, and in this case we need to call the translation function instead of a simple passthrough to the device.
Fixes piglit spec@nv_primitive_restart tests.
Fixes: 2122b902 "gallium/indices: don't expand prim-type for 8-bit indices"