Fix encoder when loopfiltering-across-tiles is enabled

Patch 5c06a646 pulled the "if (!cm->loopfilter_across_tiles_enabled)"
check out of av1_setup_across_tile_boundary_info, intending for the
condition to be checked in the caller instead. The check was added
to the decoder but not to the encoder, leading to encode/decode
mismatches when the flag was set.

Fix this by correctly applying the change to the encoder as well.

BUG=aomedia:715

Change-Id: I61182bd717e2cb84228091fb439270dce153705e