drm/xe/pf: Use migration-friendly doorbells auto-provisioning

drm/xe/pf: Use migration-friendly doorbells auto-provisioning

Instead of trying very hard to find the largest fair number of GuC
doorbell IDs that could be allocated for VFs on the current GT, pick
some smaller rounded down to power-of-two value that is more likely
to be provisioned in the same manner by the other PF instance:

  num VFs | num doorbells
  --------+--------------
   63..32 | 4
   31..16 | 8
   15..8  | 16
    7..4  | 32
    3..2  | 64
       1  | 128 (regular PF)
       1  | 240 (admin only PF)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://patch.msgid.link/20251105183253.863-3-michal.wajdeczko@intel.com