drm/client: Do not pin in drm_client_buffer_vmap()

Pin and vmap are two distict operations. Do not mix them.

The helper drm_client_buffer_vmap() maps the pages for fbdev-dma
and fbdev-shmem. In both cases, the vmap operation ensures that
the pages are available until the vunmap happens. And as the pages
in DMA or SHMEM areas cannot be moved, there is no reason to call
pin. Hence remove the pin call.

Update drm_client_buffer_vunmap() accordingly.

v2:
- call 'locked' variants of GEM helpers (Dmitry)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://lore.kernel.org/r/20250526132634.531789-2-tzimmermann@suse.de