In the bitstream in bug 709, the frame has a width of 13318 mi
units. This rounds up to 13328 mi units (when rounding to a whole
number of max superblocks) but then the shift right by log2_tile_cols
gives floor(13328/32) = floor(416.5) = 416. Of course 32*416 = 13312
which is less than 13318.

This patch moves the <mi_cols, log2_tile_cols> -> tile_width
calculation (and the equivalent calculation for tile_height) into a
common function which gets the rounding right.

BUG=aomedia:709

Change-Id: If5e8e6b7a3c6f3d336cda9edd94a17595eaf5406