drm/i915/gem: fix i915_gem_object_lookup_rcu() kernel-doc parameter name

drivers/gpu/drm/i915/gem/i915_gem_object.h:94: warning: Function parameter or member 'file' not described in 'i915_gem_object_lookup_rcu'
drivers/gpu/drm/i915/gem/i915_gem_object.h:94: warning: Excess function parameter 'filp' description in 'i915_gem_object_lookup_rcu'

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/80c96863b7a0755aaa07efb49bcccd9ba620c6d7.1683041799.git.jani.nikula@intel.com