drm/i915/gem: drop unused but set variable unpinned

Prepare for re-enabling -Wunused-but-set-variable.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6831c21567e8e84da424f32a8b7b48932803ab7b.1685119007.git.jani.nikula@intel.com