Rework of Sean's HiSi importer/planner changes

Merged John Stultz requested to merge john.stultz/drm-hwcomposer:rfc-seans-hisi-fix into master

This takes Sean's HiSi importer/planner change, and adds one small change to the planner to try to address an issue w/ the single layer case where we try to DEVICE composite a layer we only pretended to import.

Edited by John Stultz

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