allow drivers to fix kexec bus mastering hole

kexec relies on disabling bus mastering on PCI devices to block wayward DMAs left set by the previous kernel. However the drm midlayer was enabling bus mastering for all PCI drivers before calling into them. This meant no matter what they did, there was always going to be a small race window. The first patch pushes the enable into the drivers, and the second radeon patch moves bus mastering enable to later in the driver load procedure for the kms driver. These two along with Jerome's sanity patch should close the race completely.

please check the first one to make sure I didn't miss any drivers.

Dave.