drm/privacy-screen: honor acpi=off in detect_thinkpad_privacy_screen

when acpi=off is provided in bootarg, kernel crash with

[    1.252739] BUG: kernel NULL pointer dereference, address: 0000000000000018
[    1.258308] Call Trace:
[    1.258490]  ? acpi_walk_namespace+0x147/0x147
[    1.258770]  acpi_get_devices+0xe4/0x137
[    1.258921]  ? drm_core_init+0xc0/0xc0 [drm]
[    1.259108]  detect_thinkpad_privacy_screen+0x5e/0xa8 [drm]
[    1.259337]  drm_privacy_screen_lookup_init+0xe/0xe85 [drm]

The reason is that acpi_walk_namespace expects acpi related stuff
initialized but in fact it wouldn't when acpi is set to off. In this case
we should honor acpi=off in detect_thinkpad_privacy_screen().

Signed-off-by: Tong Zhang <ztong0001@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220123091004.763775-1-ztong0001@gmail.com