drm/i915: remove unused i915_gem_lmem_obj_ops declaration

i915_gem_lmem_obj_ops has been removed since
commit 213d5092 ("drm/i915/ttm: Introduce a TTM i915
gem object backend"), so remove it.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Acked-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220913024847.552254-7-cuigaosheng1@huawei.com