drm/i915/params: set i915.enable_hangcheck permissions to 0600

i915.enable_hangcheck has been an outlier since its introduction in
commit 3e0dc6b0 ("drm/i915: hangcheck disable parameter") with 0644
permissions, while all the rest are either 0400 or 0600. Follow suit
with 0600.

IGT never reads the value, so there should be no impact.

Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/5c8f7d1a1654436d38919b7419a209c129db8ad0.1545920737.git.jani.nikula@intel.com