Revert "drm/tegra: Use dma_buf from GEM object instance"

This reverts commit 482c7e2.

The dma_buf field in struct drm_gem_object is not stable over the
object instance's lifetime. The field becomes NULL when user space
releases the final GEM handle on the buffer object. This resulted
in a NULL-pointer deref.

Workarounds in commit 5307dce ("drm/gem: Acquire references on
GEM handles for framebuffers") and commit f6bfc9a ("drm/framebuffer:
Acquire internal references on GEM handles") only solved the problem
partially. They especially don't work for buffer objects without a DRM
framebuffer associated.

Hence, this revert to going back to using .import_attach->dmabuf.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
Link: https://lore.kernel.org/r/20250715084549.41473-1-tzimmermann@suse.de

Author: tdz

drivers/gpu/drm/tegra/gem.c

@@ -526,7 +526,7 @@ void tegra_bo_free_object(struct drm_gem_object *gem)
 		if (drm_gem_is_imported(gem)) {
 			dma_buf_unmap_attachment_unlocked(gem->import_attach, bo->sgt,
 							  DMA_TO_DEVICE);
-			dma_buf_detach(gem->dma_buf, gem->import_attach);
+			dma_buf_detach(gem->import_attach->dmabuf, gem->import_attach);
 		}
 	}