drm: fix the usage after free

For readdir_r(), the next directory entry is returned in caller-allocted
buffer (pointered by pent here).

https://bugs.freedesktop.org/show_bug.cgi?id=91704



Signed-off-by: Mathias Tillman <master.homer@gmail.com>
Signed-off-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>