drm/i915/perf: reconcile Excess struct member kernel-doc warnings

Document nested struct members with full names as described in
Documentation/doc-guide/kernel-doc.rst.

i915_perf_types.h:341: warning: Excess struct member 'ptr_lock' description in 'i915_perf_stream'
i915_perf_types.h:341: warning: Excess struct member 'head' description in 'i915_perf_stream'
i915_perf_types.h:341: warning: Excess struct member 'tail' description in 'i915_perf_stream'
3 warnings as Errors

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231226195432.10891-4-rdunlap@infradead.org