backend/drm: re-use previous mode blob if it didn't change
If the old mode is the same as the new one, we don't need to re-create the mode blob. This happens when the compositor calls wlr_output_set_mode() at startup, for instance.
If the old mode is the same as the new one, we don't need to re-create the mode blob. This happens when the compositor calls wlr_output_set_mode() at startup, for instance.