ice: Use ice_ena_vsi and ice_dis_vsi in DCB configuration flow

DCB configuration flow needs to disable and enable only the PF (main)
VSI, so use ice_ena_vsi and ice_dis_vsi. To avoid the use of ifdef to
control the staticness of these functions, move them to ice_lib.c.

Also replace the allocate and copy of old_cfg to kmemdup() in
ice_pf_dcb_cfg().

Signed-off-by: Anirudh Venkataramanan <anirudh.venkataramanan@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>