drm: add drm_gem_object_is_shared_for_memory_stats() helper

Add a helper so that drm drivers can consistently report
shared status via the fdinfo shared memory stats interface.

In addition to handle count, 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 inline function

Link: https://lore.kernel.org/all/20231207180225.439482-1-alexander.deucher@amd.com/
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> (v1)
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>