dma-buf: Make mmap callback actually optional

The docs state the callback is optional but it is not, make it optional.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190329165201.16233-1-afd@ti.com