Runtime: change default tiling mode to TILE_X from TILE_Y.

Runtime: change default tiling mode to TILE_X from TILE_Y.

Nanhai found that tiling mode does matter the performance much
for some cases. So make the tiling mode configurable at runtime
and make the default tiling mode as TILE_X which is much
better than the original TILE_Y for many cases.

At runtime, it is easy to change the default tiling mode as below
export OCL_TILING=0 # enable NO_TILE
export OCL_TILING=1 # enable TILE_X
export OCL_TILING=2 # enable TILE_Y

Signed-off-by: Zhigang Gong <zhigang.gong@intel.com>