MLBEDSW-8261: Fix regression on AvgPool

- When adding extended stride support for CONV_2D a
regression was introduced for AvgPool causing an
output diff for a particular test case.
- The reason was that the logic for forcing the
zero point to zero when generating the cmd stream
did not have a check for explicit padding.
- Updated logic to also include check for explicit
padding.

Change-Id: Iee4893a83a05279e592fe230f4d66d9c9ddb3e05
Signed-off-by: Johan Alfven <johan.alfven@arm.com>