dma-buf: Update [un]map documentation to match the other functions

Other function have inline documentation, a couple still have
theirs at the top of the structure, update the docs and move
them inline.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190321200957.16938-2-afd@ti.com