drivers: thermal: step-wise: Optimize mitigation request evaluation

When evaluating a thermal instance which doesn't have any previous
mitigation request and if there is no new mitigation request step wise
algorithm will still evaluate the new request. This can cause the
algorithm to trigger a false mitigation, based on the temperature trend.

Avoid evaluating the mitigation request if there is no previous and new
mitigation request for a thermal instance.

Change-Id: I056aa0640bed5f296828bfceda44204d01e93064
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>