drm: update drm_show_memory_stats() for dma-bufs

Show buffers as shared if they are shared via dma-buf as well
(e.g., shared with v4l or some other subsystem).

v2: switch to gem helper

Link: https://lore.kernel.org/all/20231207180225.439482-1-alexander.deucher@amd.com/
Reviewed-by: Rob Clark <robdclark@gmail.com> (v1)
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.keonig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>