Revert "ion: Add reserve function to ion"

This reverts commit 6f9e56945d4ee3a2621968caa72b135cb07e49c4.
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Revert "ion: Switch map/unmap dma api to sg_tables"

This reverts commit 7191e9ba2508ca6f1eaab251cf3f0a2318eebe26.

Conflicts:
	drivers/gpu/ion/ion_system_heap.c