platform/x86: intel-vbtn: Support tablet mode switch

On some laptop like the Dell Inspiron 7000 series tablet mode switch
implemented in Intel ACPI, the events to enter and exit the tablet mode
are 0xCC and 0xCD

This initializes the tablet/laptop mode at the correct value
if the system booted in tablet mode (or the intel-vbtn module
loaded with the device in tablet mode)

Cc: platform-driver-x86@vger.kernel.org
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: "Pali Rohár" <pali.rohar@gmail.com>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Mario Limonciello <mario_limonciello@dell.com>
Cc: Andy Shevchenko <andy@infradead.org>
Cc: Stefan Brüns<stefan.bruens@rwth-aachen.de>
Signed-off-by: Marco Martin <notmart@gmail.com>
Reviewed-by: Mario Limonciello <mario.limonciello@dell.com>
Acked-by: Pali Rohár <pali.rohar@gmail.com>
[andy: fixed style of comments, indentation, and massaged commit message]
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>