drm/i915: add back static declaration

warning: symbol '__i915_gem_object_create_stolen' was not declared. Should it be static?
warning: symbol '_i915_gem_object_stolen_init' was not declared. Should it be static?

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115101329.880667-1-matthew.auld@intel.com