ion: remove IS_ERR_OR_NULL

IS_ERR_OR_NULL is often part of a bad pattern that can accidentally
return 0 on error:
if (IS_ERR_OR_NULL(ptr))
    return PTR_ERR(ptr);

It also usually means that the errors of a function are not well
defined.  Replace all uses in ion.c by ensure that the return
type of any function in ion is an ERR_PTR.

Specify that the expected return value from map_kernel or map_dma
heap ops is ERR_PTR, and warn if a heap returns NULL.

Signed-off-by: Colin Cross <ccross@android.com>
Change-Id: I6e7ea0d2e62fa08d4e372a7ef6da649f7a62289c