lavapipe: fix primitive-restart for uint8 indices

Merged Erik Faye-Lund requested to merge kusma/mesa:lvp-primitive-restart-8bit-fix into master

I forgot to add an implicit primitive-restart index for uint8 while adding support for this initially. Sorry about that!

Fixes: 6526e6d1 ("lavapipe: implement VK_EXT_index_type_uint8")

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