radeonfb: Use pci_dev pm_cap

Use the already existing pm_cap variable in struct pci_dev for
determining the power management offset.  This saves the driver from
having to keep track of an extra variable.  Also, use the pci_power_t
value instead of always using the raw value for PCI_D2.

Signed-off-by: Jon Mason <jdmason@kudzu.us>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>