Calling VPC commands consists of several VPCW and VPCR ACPI calls.
These calls and their results can get mixed up if they are called
simultaneously from different threads, like acpi notify handler,
sysfs, debugfs, notification chain.

The commit e2ffcda1 ("ACPI: OSL: Allow Notify () handlers to run on
all CPUs") made the race issues much worse than before it but some
races were possible even before that commit.

Add a mutex to synchronize VPC commands.

Fixes: e2ffcda1 ("ACPI: OSL: Allow Notify () handlers to run on all CPUs")
Fixes: e82882cd ("platform/x86: Add driver for Yoga Tablet Mode switch")
Signed-off-by: Gergo Koteles <soyer@irl.hu>
Link: https://lore.kernel.org/r/f26782fa1194ad11ed5d9ba121a804e59b58b026.1721898747.git.soyer@irl.huReviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>