rust: drm: gpuvm: Add a missing lock

[WhatAmISupposedToPutHere]

Lockdep complains otherwise:

------------[ cut here ]------------
WARNING: CPU: 5 PID: 885 at drivers/gpu/drm/drm_gpuvm.c:1620 drm_gpuvm_bo_put+0x1b4/0x254
Modules linked in: brcmfmac_wcc uhid overlay squashfs zlib_inflate brcmfmac hci_bcm4377 brcmutil spi_nor aop_las aop_als industrialio cfg80211 fuse nfn>
CPU: 5 UID: 1000 PID: 885 Comm: kwin_wayland Tainted: G S W 6.18.2+ #5 PREEMPTLAZY
Tainted: [S]=CPU_OUT_OF_SPEC, [W]=WARN
Hardware name: Apple MacBook Pro (14-inch, M1 Pro, 2021) (DT)
pstate: 61400009 (nZCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--)
pc : drm_gpuvm_bo_put+0x1b4/0x254
lr : drm_gpuvm_bo_put+0x14c/0x254
sp : ffff8000893d7c10
x29: ffff8000893d7c10 x28: ffff00001b8ef9c0 x27: 0000000000000000
x26: 0000000000000002 x25: ffff800081451000 x24: dead000000000100
x23: ffff800080ee82d0 x22: ffff0000108f9d50 x21: ffff0000108f9c00
x20: ffff0000492e0700 x19: ffff000048700000 x18: 0000000000000000
x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
x14: 0000000000000008 x13: 0000000000000000 x12: ffff8000815e34d0
x11: 0000000000000001 x10: 00000000ffffffff x9 : 0000000100000000
x8 : 0000000000000000 x7 : 0000000000000000 x6 : ffff8000807d2bcc
x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
x2 : 0000000000000000 x1 : ffff000048700258 x0 : 0000000000000000
Call trace:
drm_gpuvm_bo_put+0x1b4/0x254 (P)
RINvNtCs1tcwcP3FgYC_4core3ptr13drop_in_placeINtNtNtCsgIauPoi8ikU_6kernel3drm2mm8NodeDatauNtNtCshTJcMxhWd7O_5asahi3mmu18KernelMappingInnerEEB1t+0xb0/>
RINvNtCs1tcwcP3FgYC_4core3ptr13drop_in_placeNtNtCshTJcMxhWd7O_5asahi4file2VmEBK+0xcc/0xf0
RNvMNtNtCsgIauPoi8ikU_6kernel3drm4fileINtB2_4FileNtNtCshTJcMxhWd7O_5asahi4file4FileE18postclose_callbackBP+0xac/0x2f4
drm_file_free+0x1b8/0x210
drm_release+0xb8/0x140
__fput+0xf8/0x2e4
fput_close_sync+0x44/0x114
__arm64_sys_close+0xb0/0xfc
invoke_syscall+0x48/0xc8
do_el0_svc+0x7c/0xa8
el0_svc+0x3c/0xd8
el0t_64_sync_handler+0x68/0xdc
el0t_64_sync+0x198/0x19c
---[ end trace 0000000000000000 ]---

[jannau]

manually merged to my local bits/210-gpu

iirc I tried to replace the resv lock here with the gpuva lock which resulting in the object being gone at unlock time. The dma_resv_lock probably prevents this. The safety comment is non-sense but this is going to change anyway when we rebase onto the upstream gpuvm bindings.