drm/client: Pin vmap'ed GEM buffers

The function drm_client_buffer_vmap() establishes a long-term mapping
of the client's buffer object into the kernel address space. Make sure
that buffer does not move by pinning it to its current location. Same
for vunmap with unpin.

The only caller of drm_client_buffer_vmap() is fbdev-dma, which uses
gem-dma. As DMA-backed GEM buffers do not move, this change is for
correctness with little impact in practice.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com> # virtio-gpu
Acked-by: Zack Rusin <zack.rusin@broadcom.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240227113853.8464-12-tzimmermann@suse.de