The example bitstream in bug 710 shows what might go on. This has a
frame with mi_cols=540 and log2_tile_cols=3. But 540/8 = 67.5, which
gets rounded up to give a tile_width of 5*16=80. However, 540/80 =
6.75 which means that the frame finishes somewhere in the middle of
tile 7 (and tile 8 is completely empty).

This patch clamps tile_rows/tile_cols to ensure that can't happen. The
bitstream still signals ceil(log2(tile_<rows/cols>)) so there's no
change there.

BUG=aomedia:710

Change-Id: Idfad658ef59cf71674100f5e74bd53dd192c6a84