ixgbevf: use generic power management

With legacy PM, drivers themselves were responsible for managing the
device's power states and takes care of register states.

After upgrading to the generic structure, PCI core will take care of
required tasks and drivers should do only device-specific operations.

The driver was invoking PCI helper functions like pci_save/restore_state(),
and pci_enable/disable_device(), which is not recommended.

Compile-tested only.

Signed-off-by: Vaibhav Gupta <vaibhavgupta40@gmail.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>