drm/i915/stolen: actually mark as contiguous

Stolen memory is always allocated as physically contiguous pages, so
mark the object flags as such. It looks like the flags were previously
just ignored so this had no effect. In the future we might to add the
proper plumbing for passing the flags all over the way down from the
caller, but for now we don't have a use for that.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210421104658.304142-4-matthew.auld@intel.com