drm: Replace old pointer to new idr

In the Linux kernel, the following vulnerability has been resolved:

drm: Replace old pointer to new idr

Commit 5e28b7b94408 introduced a logical error by failing to replace the
newly generated IDR pointer to old id's pointer at the correct location
within the "change handle" logic; this resulted in the issue reported by
syzbot [1].

Specifically, the new IDR object pointer is intended to replace the original
id's pointer during the normal execution flow.

Additionally, an unnecessary conditional check for the ret exit path has
been removed.

[1]
!RB_EMPTY_ROOT(&prime_fpriv->dmabufs)
WARNING: drivers/gpu/drm/drm_prime.c:224 at drm_prime_destroy_file_private+0x48/0x60 drivers/gpu/drm/drm_prime.c:224, CPU#0: syz.0.17/5833
Call Trace:
drm_file_free.part.0+0x7e6/0xcc0 drivers/gpu/drm/drm_file.c:269
drm_file_free drivers/gpu/drm/drm_file.c:237 [inline]
drm_close_helper.isra.0+0x186/0x200 drivers/gpu/drm/drm_file.c:290
drm_release+0x1ab/0x360 drivers/gpu/drm/drm_file.c:438