habanalabs: fix MMU debugfs operations

After the MMU-code refactoring, the existing MMU debugfs operations
are no longer working so we need to fix them.
In addition, remove the duplicate code that was in the debugfs code
and use the already existing MMU-code.

Signed-off-by: Moti Haimovski <mhaimovski@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>