i965/blorp/gen8: Fix blitting of interleaved msaa surfaces

i965/blorp/gen8: Fix blitting of interleaved msaa surfaces

Fixes ES31-CTS.gtf.GL31Tests.texture_stencil8.texture_stencil8_multisample.

Current logic divides given layer of one by number of samples (four)
trashing the layer to zero. Layer adjustment is only to be used with
non-interleaved msaa surfaces where samples for particular layer are
in multiple slices.

I copy-pasted a bit of documentation from
brw_blorp.c::brw_blorp_compute_tile_offsets().

Also took the opportunity to fix the comment regarding sampling
as 2D, cube textures are the only exception.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Topi Pohjolainen <topi.pohjolainen@intel.com>