MLBEDSW-7503: Avoid changing buffer index for models with only CPU ops

- When compiling a model that only contains CPU ops, Vela
unnecessary adds an empty buffer.
- This extra buffer is added due to that the fast scratch tensor
always occupies index 1.
- Since scratch and fast_scratch does not have any constant data
they can use buffer 0.

Change-Id: I25e1fb124deed7069641bde1f571b522c5bf763a
Signed-off-by: Johan Alfven <johan.alfven@arm.com>