thermal: Add mode helpers

Prepare for making the drivers not access tzd's private members.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
[staticize thermal_zone_device_set_mode()]
Signed-off-by: kernel test robot <lkp@intel.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200629122925.21729-7-andrzej.p@collabora.com