gpu: ion: Remove allocate_contiguous_ebi call for carveouts

The allocate_contiguous_ebi APIs are being deprecated in favor
of the standard dma_alloc_coherent APIs. The carveout heap
does not currently support the dma_alloc_coherent APIs so just
give a note that clients should switch over to a heap type that
does support dma_alloc_coherent for non-fixed addresses. Remove
some crufty stuff related to allocate_contiguous_ebi memory types
as well.

Change-Id: I96fd43eb59d4cde4cdfd5c8d414885467ff423db
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>