drm/gem-shmem: Add import attachment warning to locked pin function

Commit ec144244 ("drm/gem-shmem: Acquire reservation lock in GEM
pin/unpin callbacks") moved locking DRM object's dma reservation to
drm_gem_shmem_object_pin, and made drm_gem_shmem_pin_locked public, so
we need to make sure the not-imported check warning is also added to
the latter.

Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Fixes: a7802784

 ("drm/gem: Acquire reservation lock in drm_gem_{pin/unpin}()")
Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240523113236.432585-4-adrian.larumbe@collabora.com