platform/x86: intel-vbtn: Move detect_tablet_mode() to higher in the file

This is a preparation patch for calling detect_tablet_mode() from
intel_vbtn_input_setup() without needing a forward declaration.

Note this commit makes no functional changes, the moved block of code
is completely unchanged.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>