DocBook: only copy stuff to media_api if media xml is generated

It is possible to use:
	make DOCBOOKS=device-drivers.xml htmldocs

To produce just a few docbooks. In such case, the media docs
won't be built, causing the makefile target to return an error.

While this is ok for human eyes, if the above is used on an script,
it would cause troubles.

Fix it by only creating/filling the media_api directory if the
media_api.xml is found at DOCBOOKS.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>