dma-buf: headerdoc fixes

Apparently nobody noticed that dma-buf.h wasn't actually pulled into
docbook build.  And as a result the headerdoc comments bitrot a bit.
Add missing params/fields.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>