drm/i915/ttm: fix static warning

warning: symbol 'i915_gem_ttm_obj_ops' was not declared. Should it be static?

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210623143411.293630-1-matthew.auld@intel.com