Change the foreach_rtile_in_tile() function to call av1_tile_set_row/col
to calculate tile borders, rather than calculate tile borders itself.
This will then allow variable sized tiles.

Also remove some unused set tile width/height defines from aomcx.h

BUG=aomedia:844

Change-Id: I356c90cb4189b904ca1d6e57939582b2e92f3162