drm/panfrost: Add fdinfo support for memory stats

A new DRM GEM object function is added so that drm_show_memory_stats can
provide more accurate memory usage numbers.

Ideally, in panfrost_gem_status, the BO's purgeable flag would be checked
after locking the driver's shrinker mutex, but drm_show_memory_stats takes
over the drm file's object handle database spinlock, so there's potential
for a race condition here.

Signed-off-by: Adrián Larumbe <adrian.larumbe@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230929181616.2769345-4-adrian.larumbe@collabora.com