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

In the Linux kernel, the following vulnerability has been resolved:

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

This reverts commit cce16fcd7446dcff7480cd9d2b6417075ed81065.

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