drm/dp: Fix divide-by-zero regression on DP MST unplug with nouveau

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

drm/dp: Fix divide-by-zero regression on DP MST unplug with nouveau

Fix a regression when using nouveau and unplugging a StarTech MSTDP122DP
DisplayPort 1.2 MST hub (the same regression does not appear when using
a Cable Matters DisplayPort 1.4 MST hub). Trace:

divide error: 0000 [#1] PREEMPT SMP PTI
CPU: 7 PID: 2962 Comm: Xorg Not tainted 6.8.0-rc3+ #744
Hardware name: Razer Blade/DANA_MB, BIOS 01.01 08/31/2018
RIP: 0010:drm_dp_bw_overhead+0xb4/0x110 [drm_display_helper]
Code: c6 b8 01 00 00 00 75 61 01 c6 41 0f af f3 41 0f af f1 c1 e1 04 48 63 c7 31 d2 89 ff 48 8b 5d f8 c9 48 0f af f1 48 8d 44 06 ff <48> f7 f7 31 d2 31 c9 31 f6 31 ff 45 31 c0 45 31 c9 45 31 d2 45 31
RSP: 0018:ffffb2c5c211fa30 EFLAGS: 00010206
RAX: ffffffffffffffff RBX: 0000000000000000 RCX: 0000000000f59b00
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffb2c5c211fa48 R08: 0000000000000001 R09: 0000000000000020
R10: 0000000000000004 R11: 0000000000000000 R12: 0000000000023b4a
R13: ffff91d37d165800 R14: ffff91d36fac6d80 R15: ffff91d34a764010
FS: 00007f4a1ca3fa80(0000) GS:ffff91d6edbc0000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000559491d49000 CR3: 000000011d180002 CR4: 00000000003706f0
Call Trace:
<TASK>
? show_regs+0x6d/0x80
? die+0x37/0xa0
? do_trap+0xd4/0xf0
? do_error_trap+0x71/0xb0
? drm_dp_bw_overhead+0xb4/0x110 [drm_display_helper]
? exc_divide_error+0x3a/0x70
? drm_dp_bw_overhead+0xb4/0x110 [drm_display_helper]
? asm_exc_divide_error+0x1b/0x20
? drm_dp_bw_overhead+0xb4/0x110 [drm_display_helper]
? drm_dp_calc_pbn_mode+0x2e/0x70 [drm_display_helper]
nv50_msto_atomic_check+0xda/0x120 [nouveau]
drm_atomic_helper_check_modeset+0xa87/0xdf0 [drm_kms_helper]
drm_atomic_helper_check+0x19/0xa0 [drm_kms_helper]
nv50_disp_atomic_check+0x13f/0x2f0 [nouveau]
drm_atomic_check_only+0x668/0xb20 [drm]
? drm_connector_list_iter_next+0x86/0xc0 [drm]
drm_atomic_commit+0x58/0xd0 [drm]
? __pfx___drm_printfn_info+0x10/0x10 [drm]
drm_atomic_connector_commit_dpms+0xd7/0x100 [drm]
drm_mode_obj_set_property_ioctl+0x1c5/0x450 [drm]
? __pfx_drm_connector_property_set_ioctl+0x10/0x10 [drm]
drm_connector_property_set_ioctl+0x3b/0x60 [drm]
drm_ioctl_kernel+0xb9/0x120 [drm]
drm_ioctl+0x2d0/0x550 [drm]
? __pfx_drm_connector_property_set_ioctl+0x10/0x10 [drm]
nouveau_drm_ioctl+0x61/0xc0 [nouveau]
__x64_sys_ioctl+0xa0/0xf0
do_syscall_64+0x76/0x140
? do_syscall_64+0x85/0x140
? do_syscall_64+0x85/0x140
entry_SYSCALL_64_after_hwframe+0x6e/0x76
RIP: 0033:0x7f4a1cd1a94f
Code: 00 48 89 44 24 18 31 c0 48 8d 44 24 60 c7 04 24 10 00 00 00 48 89 44 24 08 48 8d 44 24 20 48 89 44 24 10 b8 10 00 00 00 0f 05 <41> 89 c0 3d 00 f0 ff ff 77 1f 48 8b 44 24 18 64 48 2b 04 25 28 00
RSP: 002b:00007ffd2f1df520 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007ffd2f1df5b0 RCX: 00007f4a1cd1a94f
RDX: 00007ffd2f1df5b0 RSI: 00000000c01064ab RDI: 000000000000000f
RBP: 00000000c01064ab R08: 000056347932deb8 R09: 000056347a7d99c0
R10: 0000000000000000 R11: 0000000000000246 R12: 000056347938a220
R13: 000000000000000f R14: 0000563479d9f3f0 R15: 0000000000000000
</TASK>
Modules linked in: rfcomm xt_conntrack nft_chain_nat xt_MASQUERADE nf_nat nf_conntrack_netlink nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 xfrm_user xfrm_algo xt_addrtype nft_compat nf_tables nfnetlink br_netfilter bridge stp llc ccm cmac algif_hash overlay algif_skcipher af_alg bnep binfmt_misc snd_sof_pci_intel_cnl snd_sof_intel_hda_common snd_soc_hdac_hda snd_sof_pci snd_sof_xtensa_dsp snd_sof_intel_hda snd_sof snd_sof_utils snd_soc_acpi_intel_match snd_soc_acpi snd_soc_core snd_compress snd_sof_intel_hda_mlink snd_hda_ext_core iwlmvm intel_rapl_msr intel_rapl_common intel_tcc_cooling x86_pkg_temp_thermal intel_powerclamp mac80211 coretemp kvm_intel snd_hda_codec_hdmi kvm snd_hda_
---truncated---