drm/prime: Unbreak virtgpu dma-buf export

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 <[email protected]>
Fixes: 207395da5a97 ("drm/prime: reject DMA-BUF attach when get_sg_table is missing")
Signed-off-by: Rob Clark <[email protected]>
Reviewed-by: Simon Ser <[email protected]>
Signed-off-by: Simon Ser <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]