drm/i915/i915_vma: Provide one missing param and demote another non-kerneldoc header

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/i915/i915_vma.c:756: warning: Function parameter or member 'ww' not described in 'i915_vma_insert'
 drivers/gpu/drm/i915/i915_vma.c:1744: warning: Function parameter or member 'vma' not described in 'i915_vma_destroy_locked'

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: David Airlie <airlied@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: intel-gfx@lists.freedesktop.org
Cc: dri-devel@lists.freedesktop.org
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Signed-off-by: Lee Jones <lee@kernel.org>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230331092607.700644-17-lee@kernel.org