yadif: restore speed of the C filtering code

Always use the special filter for the first and last 3 columns (only).

Changes made in 64ed3976 slowed the filter to just under 3/4 of what it
was.  This commit restores the speed while maintaining identical output.

For reference, on my Athlon64:
1733222 decicycles in old
2358563 decicycles in new
1727558 decicycles in this

Signed-off-by: Anton Khirnov <anton@khirnov.net>