drm/i915: un-inline i915_gem_drain_freed_objects()

I can't idenfity a single hot path that would require
i915_gem_drain_freed_objects() to be inline. Un-inline it.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6c289c55afee0d9a3067122db63277b8d60cf74f.1662390010.git.jani.nikula@intel.com