drm/prime: Unbreak virtgpu dma-buf export

virtgpu "vram" GEM objects do not implement obj->get_sg_table().  But
they also don't use drm_gem_map_dma_buf().  In fact they may not even
have guest visible pages.  But it is perfectly fine to export and share
with other virtual devices.

Reported-by: Dominik Behr <dbehr@chromium.org>
Fixes: 207395da

 ("drm/prime: reject DMA-BUF attach when get_sg_table is missing")
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/20240322214801.319975-1-robdclark@gmail.com