st/nine: Use tgsi_to_nir when preferred IR is NIR.

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.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Axel Davy <davyaxel0@gmail.com>