drm/shmem-helper: Switch drm_gem_shmem_vmap/vunmap to use pin/unpin

The vmapped pages shall be pinned in memory and previously get/put_pages()
were implicitly hard-pinning/unpinning the pages. This will no longer be
the case with addition of memory shrinker because pages_use_count > 0 won't
determine anymore whether pages are hard-pinned (they will be soft-pinned),
while the new pages_pin_count will do the hard-pinning. Switch the
vmap/vunmap() to use pin/unpin() functions in a preparation of addition
of the memory shrinker support to drm-shmem.

Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>