i965: Remove fallthrough for Gen4 cube map layout.

i965: Remove fallthrough for Gen4 cube map layout.

Now that both 2DArray and Cube layouts are taken care of by helper
functions, it's easy to just call the right function for each
generation.  This is a little cleaner than falling through.

This also reworks the comments.  Referencing "Volume 1" of the BSpec
isn't very helpful, since that's only available inside Intel, and it
doesn't even use volume numbers.  Also, "Ironlake...finally" sounds a
bit strange considering that almost all hardware uses the 2D array
approach.  At this point, Gen4 is the only special case.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>