Minor change to GPU compatibility reporting handling

Minor change to GPU compatibility reporting handling

The old code returned an int that was used to look up an
array of strings, and analysis programs can't tell whether
the use was valid.

While we could try to return an enum, instead the logic
of whether a device is compatible is separated from
describing why it isn't.

Noted TODO to move the definition of those strings to a good single
place, when there is one.

Change-Id: I504f4e7efcfcb76f97873ff1f98bee9c67b157a3