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

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

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

This reverts commit f83a9b8c7fd0557b0c50784bfdc1bbe9140c9bf8.

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