Gen11 adds support for specifying the render target index and src0
alpha present bits in the extended message descriptor. Previously,
we had to use a message header for this, requiring extra instructions
to write the fields, and two registers of extra payload.
Improves performance on my ICL 8x8 frequency locked to 700Mhz, on iris:
GfxBench5 Manhattan 3.0: 2.13635% +/- 0.159859% (n=5)
GfxBench5 Aztec Ruins: 1.57173% +/- 0.128749% (n=5)
Synmark2 OglDeferred: 2.86914% +/- 0.191211% (n=10)
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>