drm/shmem-helper: Fix unsetting shmem vaddr while vmap refcount > 0

drm/shmem-helper: Fix unsetting shmem vaddr while vmap refcount > 0

We switched to use refcount_t for vmaps and missed to change the vunmap
code to properly unset the vmap pointer, which is now cleared while vmap's
refcount > 0. Clear the cached vmap pointer only when refcounting drops to
zero to fix the bug.

Fixes: e1fc39a92332 ("drm/shmem-helper: Use refcount_t for vmap_use_count")
Reported-by: Lucas De Marchi <lucas.demarchi@intel.com>
Closes: https://lore.kernel.org/dri-devel/20250403105053.788b0f6e@collabora.com/T/#m3dca6d81bedc8d6146a56b82694624fbc6fa4c96
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Tested-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://lore.kernel.org/r/20250403142633.484660-1-dmitry.osipenko@collabora.com