pixel-formats: add name string

There is often a need to print the name of a pixel format. Printing the
raw numeric value is hard to decipher, printing the four ASCII
characters is slightly more human-friendly but still needs a decoder
table. Add a name that can be printed easily.

The bulk of this patch was done with:
sed -i -e 's/\.format = DRM_FORMAT_\(.\+\),/DRM_FORMAT(\1),/' libweston/pixel-formats.c

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>