gpu: ion: minor refactor in ion_mmap

mutex_unlock is being called in the if and the else branch. Just move
it out of the if/else. This will help reduce upcoming merge conflicts
with upstream Ion changes.

Change-Id: Ib426e73f1cb9ed79f37b9c1e0a8885066ae5b4f4
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>