drm/shmem-helpers: Redirect mmap for imported dma-buf

Currently this seems to work by converting the sgt into a pages array,
and then treating it like a native object. Do the right thing and
redirect mmap to the exporter.

With this nothing is calling get_pages anymore on imported dma-buf,
and we can start to remove the use of the ->pages array for that case.

v2: Rebase

Tested-by: Boris Brezillon <boris.brezillon@collabora.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200511093554.211493-8-daniel.vetter@ffwll.ch