Make GPE disable more robust

Implemented another change for the GPE disable. We now perform a
read-change-write of the enable register instead of simply writing out the
cached enable mask. This will prevent inadvertent enabling of GPEs if a rogue
GPE is received during initialization (before GPE handlers are installed.)

http://bugzilla.kernel.org/show_bug.cgi?id=6217

Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>
Signed-off-by: Andi Kleen <ak@linux.intel.com>