msm: kgsl: Return an error for failure in _cmdbatch_create_legacy

Callers of _cmdbatch_create_legacy expect an error to be returned in
the style of ERR_PTR.  Fix an error path wherein NULL was returned
instead.

Change-Id: Ic0dedbadcbc73f6320869c44df9599d754cd139a
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>