acpi/cstate: If C3+ is detected, BM_RLD is always set

BM_RLD is only needed on old chipsets; on new chipsets, writing to
it is a nop.  Always leave it on if C3+ is probed; this reduce I/O
accesses on idle path.
netmap: bring back cdevpriv usage

This partially reverts commit 3efed4582e1 now that we have these
functions.  There are three remaining porting issues:

* vm fake pages support
* poll to kqfilter
* if_transmit
netmap: remove bogus NULL check

Reported-by: swildner
doc: remove cdevpriv bits from porting guide
acpi/cstate: Fix up comment
acpi/cstate: Go though set_lowest routing after _CST reprobing

So C3+ state transition could be properly recorded and perper cputimer
could be selected.