UPSTREAM: drm/virtio: Replace ttm_bo_unref with ttm_bo_put

The function ttm_bo_put releases a reference to a TTM buffer object. The
function's name is more aligned to the Linux kernel convention of naming
ref-counting function _get and _put.

A call to ttm_bo_unref takes the address of the TTM BO object's pointer and
clears the pointer's value to NULL. This is not necessary in most cases and
sometimes even worked around by the calling code. A call to ttm_bo_put only
releases the reference without clearing the pointer.

The current behaviour of cleaning the pointer is kept in the calling code,
but should be removed if not required in a later patch.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20180731062127.10131-3-tzimmermann@suse.de


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
(cherry picked from commit 0e7a3d4b66e18c7a56124594b124d0337059c4f2)
Signed-off-by: Greg Hartman <ghartman@google.com>
BUG: 139386237
Change-Id: I014fa5f259fafd3b8ea691adc993eaffc46dd5dc