st/nine: Use tgsi_to_nir when preferred IR is NIR.
This patch allows nine to read the preferred IR from pipe caps and use NIR when that is preferred by the driver, by calling tgsi_to_nir. Also adds some debug options that allow overriding it.
This is beneficial for a couple of reasons:
- nine will use NIR when it's enabled in radeonsi, which may result in better performance for some games
- nine will work with drivers that don't want to use
tgsi_to_nir
or are planning to stop using it in the future