ACPI / EC: Fix unexpected ec_remove_handlers() invocations

The ec_remove_handlers() is invoked without checking
EC_FLAGS_HANDLERS_INSTALLED, this patch enhances this check to avoid issues
that acpi_disable_gpe() is invoked unexpectedly to reduce the GPE runtime
count. This may happen when the EC handler installation failed on some
platforms.

Reported-by: Venkat Raghavulu <venkat.raghavulu@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>