drm/shmem-helper: Don't use vmap_use_count for dma-bufs

There is no need to refcount vmappings of dma-bufs because dma-buf core
has its own refcounting. Drop the refcounting of dma-bufs. This will ease
replacing of all drm-shmem locks with a single dma-buf reservation lock,
preparing drm-shmem code for addition of the generic drm-shmem shrinker.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>