platform/x86: thinkpad_acpi: Add support for X1 Yoga (2016) Tablet Mode

For whatever reason, the X1 Yoga doesn't support the normal method of
querying for tablet mode. Instead of providing the MHKG method under the
hotkey handle, we're instead given the CMMD method under the EC handle.
Values on this handle are either 0x1, laptop mode, or 0x6, tablet mode.

Tested-by: Daniel Martin <consume.noise@gmail.com>
Signed-off-by: Lyude <lyude@redhat.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>