i965/fs: Don't use Gen7-style scratch block reads on Gen9+.

i965/fs: Don't use Gen7-style scratch block reads on Gen9+.

Unfortunately Gen7 scratch block reads and writes seem to be hardwired
to BTI 255 even on Gen9+ where that index causes the dataport to do an
IA-coherent read or write.  This change is required for the next patch
to be correct, since otherwise we would be writing to the scratch
space using non-coherent access and then reading it back using
IA-coherent reads, which wouldn't be guaranteed to return the value
previously written to the same location without introducing an
additional HDC flush in between.

Reviewed-by: Kristian Høgsberg  <krh@bitplanet.net>