avfilter/vf_delogo: Use AVPixFmtDescriptor.nb_components

Relying on AVPixFmtDescriptor.nb_components is cleaner and faster than
checking data and linesize for every possible plane.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
