Make avcodec_pix_fmt_string() use the information in the pixel format

Make avcodec_pix_fmt_string() use the information in the pixel format
descriptors for printing the number of channels/components.

Also replace the term "nb_channels" with "nb_components" which is more
consistent with the FFmpeg internal terminology, and is somehow
different with respect to the current definition of nb_channels in
PixFmtInfo.

See thread:
Subject: [FFmpeg-devel] [PATCH 6/8] Make avcodec_pix_fmt_string() use the
        information in the pixel format descriptors for printing the
        number of planes. Also replace the term "nb_channels" with
        "nb_planes" which is more correct.
Date: Fri,  5 Nov 2010 12:01:38 +0100

Originally committed as revision 25717 to svn://svn.ffmpeg.org/ffmpeg/trunk