hp-wmi: Add support for reporting tablet state

HP tablets send a WMI event when a tablet state change occurs, but use the
same method as is used for reporting docking and undocking. The same query
is used to obtain the state of the hardware. Bit 0 indicates the docking
state, while bit 2 indicates the tablet state. This patch breaks these out
and sends separate input events for tablet and dock state changes. An
additional sysfs file is added to report the tablet state.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Len Brown <len.brown@intel.com>