After thermal zone device registered, as we have not read any
temperature before, thus tz->temperature should not be 0,
which actually means 0C, and thermal trend is not available.
In this case, we need specially handling for the first
thermal_zone_device_update().

Both thermal core framework and step_wise governor is
enhanced to handle this. And since the step_wise governor
is the only one that uses trends, so it's the only thermal
governor that needs to be updated.

CC: <stable@vger.kernel.org> #3.18+
Tested-by: NManuel Krause <manuelkrause@netscape.net>
Tested-by: Nszegad <szegadlo@poczta.onet.pl>
Tested-by: Nprash <prash.n.rao@gmail.com>
Tested-by: Namish <ammdispose-arch@yahoo.com>
Tested-by: NMatthias <morpheusxyz123@yahoo.de>
Reviewed-by: NJavi Merino <javi.merino@arm.com>
Signed-off-by: NZhang Rui <rui.zhang@intel.com>
Signed-off-by: NChen Yu <yu.c.chen@intel.com>