drm/i915/vma: constify unbind_fence_ops

unbind_fence_ops can be const and placed in rodata.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Jouni Högander <jouni.hogander@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230818112758.3586545-1-jani.nikula@intel.com