intel_idle: mark few variables as __read_mostly

The intention is to clean up the code and make it look a bit more
consistent.

Mark all unitialized module parameter variables as __read_mostly,
not just one of them. The other parameters are read-mostly too.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>