i965: Set the "Float Blend Optimization Enable" bit on Gen9+.

i965: Set the "Float Blend Optimization Enable" bit on Gen9+.

This is woefully undocumented.  It's some kind of optimization that
avoids unnecessary render target reads when blending with a floating
point render target, using independent alpha blending modes.

The internal documentation indicates that this bit exists on Cherryview
as well, but the other driver doesn't appear to set it on that platform.
There's also some confusing wording that indicates that it may exist on
Broadwell, but the documentation says it's reserved, so who knows.

I was not able to find any workload that benefited from setting this
bit, but it seems like a good idea to set it nonetheless.

Reviewed-by: Plamena Manolova <plamena.manolova@intel.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>