gpu: ion: Return error value when iommu map fails

When __ion_map_iommu returns a non-zero value (i.e.
error occured), properly propagate this error back
up to the client.

Change-Id: I6cd5ce8a723ad1cbbd6c42b712f5a19fa2ba719f
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>