iris: Map FXT1 texture formats

iris: Map FXT1 texture formats

This exposes GL_TDFX_texture_compression_FXT1 support.  It's ancient,
only Intel GPUs appear to support it, and I seriously doubt anybody
uses it.  But i965 supports it, and it's trivial to do, so we may as
well support it in the new iris driver as well.

Reviewed-by: Eric Anholt <eric@anholt.net>