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

drivers/gpu/drm/drm_prime.c

@@ -582,7 +582,12 @@ int drm_gem_map_attach(struct dma_buf *dma_buf,
 {
 	struct drm_gem_object *obj = dma_buf->priv;
 
-	if (!obj->funcs->get_sg_table)
+	/*
+	 * drm_gem_map_dma_buf() requires obj->get_sg_table(), but drivers
+	 * that implement their own ->map_dma_buf() do not.
+	 */
+	if (dma_buf->ops->map_dma_buf == drm_gem_map_dma_buf &&
+	    !obj->funcs->get_sg_table)
 		return -ENOSYS;
 
 	return drm_gem_pin(obj);