isl: Finish tiling filtering for Gen6.

isl: Finish tiling filtering for Gen6.

Gen6 only has one additional restriction over Gen7+, so we just add it
to the existing gen7 function (which actually covers later gens too).

This should stop FINISHME spew when running GL on Sandybridge.

v2: Fix bytes per block vs. bits per block confusion (Jason) and
    rename function to gen6_filter_tiling (Jason and Chad).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>