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

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

This reverts commit 1a148af06000e545e714fe3210af3d77ff903c11.

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 5307dce878d4 ("drm/gem: Acquire references on
GEM handles for framebuffers") and commit f6bfc9afc751 ("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