drm/shmem-helper: Switch to use drm_* debug helpers

Ease debugging of a multi-GPU system by using drm_WARN_*() and
drm_dbg_kms() helpers that print out DRM device name corresponding
to shmem GEM.

Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Suggested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://lore.kernel.org/all/20230108210445.3948344-6-dmitry.osipenko@collabora.com/