thermal: fair_share: use the weight from the thermal instance

The fair share governor is not usable with thermal zones that use the
bind op and don't populate thermal_zone_parameters, the majority of
them.  Now that the weight is in the thermal instance, we can use that
in the fair share governor to allow every thermal zone to trivially use
this governor.  Furthermore, this simplifies the code.

Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Durgadoss R <durgadoss.r@intel.com>
Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
(cherry picked from commit 8b91e2cb24fbba3825d09a412ce2e1966e7c92d1)
Signed-off-by: Kevin Hilman <khilman@linaro.org>