drm/i915: fix null pointer dereference

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

drm/i915: fix null pointer dereference

Asus chromebook CX550 crashes during boot on v5.17-rc1 kernel.
The root cause is null pointer defeference of bi_next
in tgl_get_bw_info() in drivers/gpu/drm/i915/display/intel_bw.c.

BUG: kernel NULL pointer dereference, address: 000000000000002e
PGD 0 P4D 0
Oops: 0002 [#1] PREEMPT SMP NOPTI
CPU: 0 PID: 1 Comm: swapper/0 Tainted: G U 5.17.0-rc1
Hardware name: Google Delbin/Delbin, BIOS Google_Delbin.13672.156.3 05/14/2021
RIP: 0010:tgl_get_bw_info+0x2de/0x510
...
[ 2.554467] Call Trace:
[ 2.554467] <TASK>
[ 2.554467] intel_bw_init_hw+0x14a/0x434
[ 2.554467] ? _printk+0x59/0x73
[ 2.554467] ? _dev_err+0x77/0x91
[ 2.554467] i915_driver_hw_probe+0x329/0x33e
[ 2.554467] i915_driver_probe+0x4c8/0x638
[ 2.554467] i915_pci_probe+0xf8/0x14e
[ 2.554467] ? _raw_spin_unlock_irqrestore+0x12/0x2c
[ 2.554467] pci_device_probe+0xaa/0x142
[ 2.554467] really_probe+0x13f/0x2f4
[ 2.554467] __driver_probe_device+0x9e/0xd3
[ 2.554467] driver_probe_device+0x24/0x7c
[ 2.554467] __driver_attach+0xba/0xcf
[ 2.554467] ? driver_attach+0x1f/0x1f
[ 2.554467] bus_for_each_dev+0x8c/0xc0
[ 2.554467] bus_add_driver+0x11b/0x1f7
[ 2.554467] driver_register+0x60/0xea
[ 2.554467] ? mipi_dsi_bus_init+0x16/0x16
[ 2.554467] i915_init+0x2c/0xb9
[ 2.554467] ? mipi_dsi_bus_init+0x16/0x16
[ 2.554467] do_one_initcall+0x12e/0x2b3
[ 2.554467] do_initcall_level+0xd6/0xf3
[ 2.554467] do_initcalls+0x4e/0x79
[ 2.554467] kernel_init_freeable+0xed/0x14d
[ 2.554467] ? rest_init+0xc1/0xc1
[ 2.554467] kernel_init+0x1a/0x120
[ 2.554467] ret_from_fork+0x1f/0x30
[ 2.554467] </TASK>
...
Kernel panic - not syncing: Fatal exception

(cherry picked from commit c247cd03898c4c43c3bce6d4014730403bc13032)