platform/x86: thinkpad_acpi: convert mutex_lock() to guard(mutex)

Convert straightforward mutex_lock() and mutex_unlock() usages for
hotkey_mutex, tpacpi_inputdev_send_mutex, kbdlight_mutex, lcdshadow_dev
lock, and dytc_mutex to guard(mutex) and scoped_guard(mutex) helpers
from linux/cleanup.h.

This improves code readability and ensures that mutexes are
automatically released when exiting their respective scopes.

Assisted-by: Antigravity:gemini-3.6-flash
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Tested-by: Ali Ahmet Memiş <aliamemis@disroot.org>
Reviewed-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Tested-by: Mark Pearson <mpearson-lenovo@squebb.ca>
Link: https://patch.msgid.link/20260806061925.625482-1-dmitry.torokhov@gmail.com
[ij: converted hotkey_poll_setup() as well]
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>