Revert "drm/i915/perf: Fix warning in documentation"

Userspace portion is still missing.

This reverts commit 9fa6e2f7

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Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Acked-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181116135510.13807-2-joonas.lahtinen@linux.intel.com