ACPICA: GPE detect optimization - ignore unused GPE registers

This optimization will simply ignore GPE registers that contain
no enabled GPEs - there is no need to read the register.
ACPICA bugzilla 884.

http://www.acpica.org/bugzilla/show_bug.cgi?id=884

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