Add shared stats.  Useful for seeing shared memory.

v2: take dma-buf into account as well
v3: use the new gem helper

Link: https://lore.kernel.org/all/20231207180225.439482-1-alexander.deucher@amd.com/Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Christian König <christian.keonig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>