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

Fixes: c64a9a7c ("drm/i915: Update memory bandwidth formulae")
Signed-off-by: Łukasz Bartosik <lb@semihalf.com>
Reviewed-by: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220201153354.11971-1-lukasz.bartosik@semihalf.com