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 <[e-mail]>
Reviewed-by: Hans de Goede <[e-mail]>
Signed-off-by: Hans de Goede <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_privacy_screen_x86.c

···
 	unsigned long long output;
 	acpi_status status;
 
+	if (acpi_disabled)
+		return false;
+
 	/* Get embedded-controller handle */
 	status = acpi_get_devices("PNP0C09", acpi_set_handle, NULL, &ec_handle);
 	if (ACPI_FAILURE(status) || !ec_handle)