UPSTREAM: PM: EM: Check if the get_cost() callback is present in em_compute_costs()

Subsequent changes will introduce a case in which 'cb->get_cost' may
not be set in em_compute_costs(), so add a check to ensure that it is
not NULL before attempting to dereference it.

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

(cherry picked from commit 81886722)
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Change-Id: Ica074a46b7f1fd0b19b86b10a9c14aa6e03c40c1