TIFF: fix plane count calculation for pyramids with multiple planes

Fixes https://trac.openmicroscopy.org.uk/ome/ticket/12617.  To test, import the files from QA 10321 and verify that all Z sections (7 for the larger files, 1 for the smaller file) can be viewed without an exception in Blitz-0.log after pyramid generation completes.  Pyramids generated without this change should be read correctly as well, so reimport should not be strictly necessary if all files were previously imported into a develop server.

/cc @jburel, @zeb