drm/xe/vf: Improve getting clean NULL context

drm/xe/vf: Improve getting clean NULL context

There is a small risk that when fetching a NULL context image the
VF may get a tweaked context image prepared by another VF that was
previously running on the engine before the GuC scheduler switched
the VFs.

To avoid that risk, without forcing GuC scheduler to trigger costly
engine reset on every VF switch, use a watchdog mechanism that when
configured with impossible condition, triggers an interrupt, which
GuC will handle by doing an engine reset. Also adjust job size to
account for additional dwords with watchdog setup.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Michał Winiarski <michal.winiarski@intel.com>
Link: https://patch.msgid.link/20260303201354.17948-4-michal.wajdeczko@intel.com