tests/i915/gem_ctx_persistence: fix gcc warning

casting unsigned char pointer to int pointer causes gcc to be unhappy with
comment: "warning: dereferencing type-punned pointer will break
strict-aliasing rules"

Signed-off-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Petri Latvala <petri.latvala@intel.com>