platform/x86: intel-vbtn: Only activate tablet mode switch on 2-in-1's

Some laptops such as the XPS 9360 support the intel-vbtn INT33D6
interface but don't initialize the bit that intel-vbtn uses to
represent switching tablet mode.

By running this only on real 2-in-1's it shouldn't cause false
positives.

Fixes: 30323fb6

 ("Support tablet mode switch")
Reported-by: Jeremy Cline <jeremy@jcline.org>
Signed-off-by: Mario Limonciello <mario.limonciello@dell.com>
Tested-by: Jeremy Cline <jeremy@jcline.org>
Tested-by: Darren Hart (VMware) <dvhart@infradead.org>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>