drm/shmem-helper: Change sgt allocation policy

In a preparation to addition of drm-shmem memory shrinker support, change
the SGT allocation policy in this way:

1. SGT can be allocated only if shmem pages are pinned at the
time of allocation, otherwise allocation fails.

2. Drivers must ensure that pages are pinned during the time of SGT usage
and should get new SGT if pages were unpinned.

This new policy is required by the shrinker because it will move pages
to/from SWAP unless pages are pinned, invalidating SGT pointer once pages
are relocated.

Previous patches prepared drivers to the new policy.

Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>