intel/fs: Use split sends for surface writes on gen9+

intel/fs: Use split sends for surface writes on gen9+

Surface reads don't need them because they just have the one address
payload.  With surface writes, on the other hand, we can put the address
and the data in the different halves and avoid building the payload all
together.

The decrease in register pressure and added freedom in register
allocation resulting from this change reduces spilling enough to improve
the performance of one customer benchmark by about 2x.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>