platform/x86: thinkpad_acpi: re-initialize ACPI buffer size when reuse

Evaluating ACPI _BCL could fail, then ACPI buffer size will be set to 0.
When reuse this ACPI buffer, AE_BUFFER_OVERFLOW will be triggered.

Re-initialize buffer size will make ACPI evaluate successfully.

Fixes: 46445b6b ("thinkpad-acpi: fix handle locate for video and query of _BCL")
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>