gallium: Use R{8,16,32}_UINT for querying index buffer formats
Consensus in !10973 (merged) seems to be:
- Gallium doesn't really care about index buffer formats but some drivers do, for
is_format_supported
checks - There's no natural reason to use I*_UINT formats for this, since those are for intensity formats.
- That a bunch of drivers did anyway is the result of years of cargo culting, not engineering consensus.
- R*_UINT formats are the natural choice for drivers that need to pick one.
So let's fix up the tree, document the convention, and unblock @zmike 's Zink patches!