drm_gem_map_dma_buf() requires drm_gem_object_funcs.get_sg_table
to be implemented, or else WARNs.

Allow drivers to leave this hook unimplemented to implement purely
local DMA-BUFs (ie, DMA-BUFs which cannot be imported anywhere
else but the device which allocated them). In that case, reject
imports to other devices in drm_gem_map_attach().

v2: new patch

v3: use ENOSYS
Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Tian Tao <tiantao6@hisilicon.com>
Cc: Maxime Ripard <maxime@cerno.tech>
Cc: Hans de Goede <hdegoede@redhat.com>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230302143502.500661-1-contact@emersion.fr