drm/i915/selftests: Fix engine reset count storage for multi-tile

Engine->id namespace is per-tile so struct igt_live_test->reset_engine[]
needs to be two-dimensional so engine reset counts from all tiles can be
stored with no aliasing. With aliasing, if we had a real multi-tile
platform, the reset counts would be incorrect for same engine instance on
different tiles.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 0c29efa2 ("drm/i915/selftests: Consider multi-gt instead of to_gt()")
Reported-by: Alan Previn Teres Alexis <alan.previn.teres.alexis@intel.com>
Cc: Tejas Upadhyay <tejas.upadhyay@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231201122109.729006-1-tvrtko.ursulin@linux.intel.com