video: fbdev: offb: Call pci_enable_device() before using the PCI VGA device

Currently the offb module will use the PCI VGA device as frame
buffer device without calling something like pci_enable_device().
However, this would cause some problem if we disable memory
decoding of the upstream bridge before. When the console driver
issued memory access to the VGA device, the access cannot be
supported by the bridge which will cause EEH error on Power machine.

Signed-off-by: Yongji Xie <xyjxie@linux.vnet.ibm.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>