drm/xe/pf: Fix VF FLR synchronization between all GTs

drm/xe/pf: Fix VF FLR synchronization between all GTs

If subsequent VF FLR request is triggered when previous VF FLR
sequence is still being processed, we ignore it as not needed.

But in case of the multi-GT platforms, one GT may already finish
its VF FLR processing and will start a new sequence, which includes
new cross-GT synchronization point.  However, since other GT may
be still busy with post-sync cleanup steps, this will put on hold
this new FLR sequence, which might never finish due to lack of any
future synchronization checkouts.

Add additional cross-GT FLR synchronization point when each GT
ends processing its own FLR sequence.  This should also help to
cover the case when one GT fails FLR processing before reaching
the first synchronization point.

Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6287
Fixes: 2a8fcf7cc950 ("drm/xe/pf: Synchronize VF FLR between all GTs")
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://patch.msgid.link/20251025124906.5264-1-michal.wajdeczko@intel.com