dma-buf: Remove unneeded sanity checks

ops, ops->map_dma_buf and ops->unmap_dma_buf are guaranteed to be
non-NULL by a check in dma_buf_export(). Remove NULL checks on those
variables in the other API functions.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>