drm/i915/userptr: Don't mark readonly objects as dirty

If we map an object as readonly into the GTT, we know that the GPU
cannot have written to it and so the object is not dirty and we don't
need to flush the writes back to the system.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190709081718.27843-1-chris@chris-wilson.co.uk