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

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

Instead of trying very hard to find the largest fair VRAM (aka LMEM)
size that could be allocated for VFs on the current tile, 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 instances.

In some cases, the outcome of above calculation might not be optimal,
but it's expected that admin will do fine-tuning using sysfs files.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://patch.msgid.link/20260218205553.3561-6-michal.wajdeczko@intel.com