dma-buf: Remove requirement for ops->map() from dma_buf_export

Since commit 9ea0dfbf972 ("dma-buf: make map_atomic and map function
pointers optional"), the core provides the no-op functions when map and
map_atomic are not provided, so we no longer need assert that are
supplied by a dma-buf exporter.

Fixes: 09ea0dfb ("dma-buf: make map_atomic and map function pointers optional")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20180807183647.22626-1-chris@chris-wilson.co.uk


Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>