Fix reading of pixels in (4 and 8-plane) planar modes.

There seems to be no alternative to reading each plane independently using
3 slow i/o's per plane (this delivers 8 nearby pixels, but we don't buffer
the results so run 8 times slower than necessary.

All the code for this was there, but it was ifdefed out and replaced by
simpler code that cannot work in planar modes. The ifdefed out code
was correct except it was missing a volatile declaration, so compilers
optimized the multiple dummy reads in it to a single read.