dma-buf: Remove leftover [un]map_atomic comments

The map_atomic/unmap_atomic callbacks have been removed, remove
the related comments.

Fixes: f664a526 ("dma-buf: remove kmap_atomic interface")
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/20190321200957.16938-1-afd@ti.com