drm/i915/perf: make oa format tables const

No reason not to be const.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180613114929.14541-1-jani.nikula@intel.com