This is just a code cleanup, moving the calculation of cm->tile_rows
and cm->tile_cols into get_tile_size, which is already calculating
cm->tile_width and cm->tile_height. The patch also gets rid of a
spurious AOMMIN and replaces it with an assertion.

Change-Id: I46666c4a197ac26d4b3746d9ea6575dc4af0d570