Revert "drm/gem-dma: Use dma_buf from GEM object instance"

This reverts commit e8afa15.

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.

v3:
- cc stable

Signed-off-by: Thomas Zimmermann <[email protected]>
Reviewed-by: Simona Vetter <[email protected]>
Acked-by: Christian König <[email protected]>
Acked-by: Zack Rusin <[email protected]>
Cc: <[email protected]> # v6.15+
Link: https://lore.kernel.org/r/[email protected]

Author: Thomas Zimmermann

drivers/gpu/drm/drm_gem_dma_helper.c

@@ -230,7 +230,7 @@ void drm_gem_dma_free(struct drm_gem_dma_object *dma_obj)
 
 	if (drm_gem_is_imported(gem_obj)) {
 		if (dma_obj->vaddr)
-			dma_buf_vunmap_unlocked(gem_obj->dma_buf, &map);
+			dma_buf_vunmap_unlocked(gem_obj->import_attach->dmabuf, &map);
 		drm_prime_gem_destroy(gem_obj, dma_obj->sgt);
 	} else if (dma_obj->vaddr) {
 		if (dma_obj->map_noncoherent)