intel/fs: Add explicit last_rt flag to fb writes orthogonal to eot.

intel/fs: Add explicit last_rt flag to fb writes orthogonal to eot.

When using multiple RT write messages to the same RT such as for
dual-source blending or all RT writes in SIMD32, we have to set the
"Last Render Target Select" bit on all write messages that target the
last RT but only set EOT on the last RT write in the shader.
Special-casing for dual-source blend works today because that is the
only case which requires multiple RT write messages per RT.  When we
start doing SIMD32, this will become much more common so we add a
dedicated bit for it.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>