gpu: ion: Return correct error code at alloc fail

If we fail to allocate a page from the iommu heap return
correct error code to ensure caller will not use pagees
that did not get allocated.

CRs-fixed: 448796
Change-Id: I0ce086608dc1c97ccf203bcbd2a0baaee905a461
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>