Prevent left_block_mode stepping into left tile

Prevent left_block_mode stepping into left tile

This commit uses left_available flag to decide if the left mode_info
struct is available for left_block_mode. As discussed with James
Zern (jzern@), this prevents the codec from fetching mode_info from
blocks in the left tile, which although effectively not used might
present concerns for multi-threaded tile decoding.

This is NOT a bit-stream change.

Change-Id: I1dc8cf1bcbf056688eee27c7bc5706ac4b4e0125