gpu: ion: Remove unnecessary error handling

Ion core takes care of the case failing allocaion. Remove unnecessary
code and obsolete comment.

Change-Id: I0a27f6697beb1b3e802275eb01bfdc4fc0ae3ae2
Signed-off-by: Hiroshi DOYU <hdoyu@nvidia.com>
Reviewed-on: http://git-master/r/75533
Reviewed-by: Automatic_Commit_Validation_User
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Varun Wadekar <vwadekar@nvidia.com>