media: pixfmt-packed-yuv.rst: fix PDF output

Some lines inside this table has more than 34 columns, which isn't
what is expected, according to the .. tabularcolumns:: definition.

That causes the PDF output to fail.

This patch ensures that all columns will have exactly the same
size, merging the unused ones using :cpan:

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>