drm/xe/pf: Increase PF GuC Buffer Cache size and use it for VF migration

drm/xe/pf: Increase PF GuC Buffer Cache size and use it for VF migration

Contiguous PF GGTT VMAs can be scarce after creating VFs.
Increase the GuC buffer cache size to 8M for PF so that we can fit GuC
migration data (which currently maxes out at just over 4M) and use the
cache instead of allocating fresh BOs.

Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patch.msgid.link/20251112132220.516975-13-michal.winiarski@intel.com
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>