docs/driver-api: Apply changed source file names

Few files under dma-buf/ changed their names but the changes didn't
applied to a document that referencing them.  It is causing few
documentation build warnings.  This commit fixes the problems by
applying changed file names on the document.

Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>