powerpc: refactor dmamap setup; free dmamap in error paths
- refactor the dmamap allocation / setup path for future code changes to align with arm64 busdma code;
- free the dmamap allocation if the dma segment list can't be allocated;
- free the dmamap allocation during the busdma dmamem allocation path if the actual memory allocation itself fails.
Locally tested:
- POWER9 ppc64le native boot, Raptor Engineering Blackbird
- POWER8 ppc6le, IBM POWER S822LC