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

JIRA: https://issues.redhat.com/browse/RHEL-106712

Conflicts:

The following hunk didn't apply:

```
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -351,7 +351,7 @@ int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem,
 	dma_resv_assert_held(obj->resv);

 	if (drm_gem_is_imported(obj)) {
-		ret = dma_buf_vmap(obj->dma_buf, map);
+		ret = dma_buf_vmap(obj->import_attach->dmabuf, map);
 	} else {
 		pgprot_t prot = PAGE_KERNEL;
```

Because the function drm_gem_shmem_vmap() was refactored to
drm_gem_shmem_vmap_locked() upstream, but we don't have the changes.
I apply only the relevant changes to both dma_buf_vmap() calls present
in drm_gem_shmem_vmap().

commit 6d496e9
Author: Thomas Zimmermann <tzimmermann@suse.de>
Date:   Tue Jul 15 17:58:16 2025 +0200

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

    This reverts commit 1a148af.

    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 <tzimmermann@suse.de>
    Reviewed-by: Simona Vetter <simona.vetter@ffwll.ch>
    Acked-by: Christian König <christian.koenig@amd.com>
    Acked-by: Zack Rusin <zack.rusin@broadcom.com>
    Cc: <stable@vger.kernel.org> # v6.15+
    Link: https://lore.kernel.org/r/20250715155934.150656-7-tzimmermann@suse.de

Signed-off-by: José Expósito <jexposit@redhat.com>

Author: jexposit

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -339,10 +339,10 @@ int drm_gem_shmem_vmap(struct drm_gem_shmem_object *shmem,
 	int ret = 0;
 
 	if (drm_gem_is_imported(obj)) {
-		ret = dma_buf_vmap(obj->dma_buf, map);
+		ret = dma_buf_vmap(obj->import_attach->dmabuf, map);
 		if (!ret) {
 			if (drm_WARN_ON(obj->dev, map->is_iomem)) {
-				dma_buf_vunmap(obj->dma_buf, map);
+				dma_buf_vunmap(obj->import_attach->dmabuf, map);
 				return -EIO;
 			}
 		}
@@ -405,7 +405,7 @@ void drm_gem_shmem_vunmap(struct drm_gem_shmem_object *shmem,
 	struct drm_gem_object *obj = &shmem->base;
 
 	if (drm_gem_is_imported(obj)) {
-		dma_buf_vunmap(obj->dma_buf, map);
+		dma_buf_vunmap(obj->import_attach->dmabuf, map);
 	} else {
 		dma_resv_assert_held(shmem->base.resv);