Rework pci_set_power_state() so that the platform callback is
invoked before the native mechanism, if necessary.  Also, make
the function check if the device is power manageable by the
platform before invoking the platform callback.

This may matter if the device dependent on additional power
resources controlled by the platform is being put into D0, in which
case those power resources must be turned on before we attempt to
handle the device itself.
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>