[media] DocBook/media/Makefile: Do not fail mkdir if dir already exists

Commit 5240f4e6

 ("[media] DocBook/media/Makefile: Avoid make htmldocs
to fail") introduced a mkdir which is always called through
install_media_images from the Documentation/DocBook/Makefile htmldocs rule.
If you run 'make htmldocs' more than once you get:

 mkdir: cannot create directory ‘./Documentation/DocBook//media_api’:
  File exists

Add -p to the mkdir to continue no matter if the dir already exists.

Signed-off-by: Graham Whaley <graham.whaley@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>