radeon_suspend_kms: don't mess with pci state that's managed by the bus

radeon_suspend_kms: don't mess with pci state that's managed by the bus

The pci bus driver handles the power state, it also manages
configuration state saving and restoring for its child devices.  Thus a
PCI device driver does not have to worry about those things.  In fact, I
observe a hard system hang when trying to suspend a system with active
radeonkms driver where both the bus driver and radeonkms driver try to
do the same thing.  I suspect that it could be because of an access to a
PCI configuration register after the device is placed into D3 state.

Reviewed by:	dumbbell, jhb
MFC after:	13 days
Differential Revision:	https://reviews.freebsd.org/D3561