ixgbe: consolidate packet buffer allocation

Consolidate packet buffer allocation currently being
done in the DCB path and main path. This allows the
feature set and packet buffer requirements to be done
once.

This is prep work to allow DCB to coexist with other
features namely, flow director.

CC: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>