drm/i915/perf: Save/restore EU flex counters across reset

If a drm client is killed, then hw contexts used by the client are reset
immediately. This reset clears the EU flex counter configuration. If an
OA use case is running in parallel, it would start seeing zeroed eu
counter values following the reset even if the drm client is restarted.
Save/restore the EU flex counter config so that the EU counters can be
monitored continuously across resets.

v2:
- Save/restore eu flex config only for gen12, as for pre-gen12, these
  are saved and restored in the context image.

Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221026222102.5526-14-umesh.nerlige.ramappa@intel.com