IVGCVSW-3223 Fix ref convolution performance regression

IVGCVSW-3223 Fix ref convolution performance regression

 * Do not use DataLayoutIndexed::GetIndex for weights and inputs,
   as it causes a large regression in performance
 * It turned out that the calculation of the indexes for the weights
   and inputs was the way it was because of an optimization done
   many months ago
 * Reverted the relevant hunks and added some comments so
   we won't make the same mistake again in the future
 * Made the GetIndex function inline to speed up other usages

Change-Id: I343b2ef0446993086f58b9dea1f0de0ba2d92216
Signed-off-by: Matteo Martincigh <matteo.martincigh@arm.com>