drm/xe/pf: Add FLR_PREPARE state to VF control flow

drm/xe/pf: Add FLR_PREPARE state to VF control flow

Our xe-vfio-pci component relies on the confirmation from the PF
that VF FLR processing has finished, but due to the notification
latency on the HW/FW side, PF might be unaware yet of the already
triggered VF FLR.

Update VF state machine with new FLR_PREPARE state that indicate
imminent VF FLR notification and treat that as a begin of the FLR
sequence. Also introduce function that xe-vfio-pci should call to
guarantee correct synchronization.

v2: move PREPARE into WIP, update commit msg (Michal)

Signed-off-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Co-developed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patch.msgid.link/20260309152449.910636-2-piotr.piorkowski@intel.com
Signed-off-by: Michał Winiarski <michal.winiarski@intel.com>