msm: kgsl: Check the return value from ocmem_allocate for error

ocmem_allocate() will soon be changing to return an error code in
some situations. Use IS_ERR_OR_NULL() to avoid the embarrassment of
dereferencing the result of ERR_PTR() by mistake.

Change-Id: Ic0dedbadbb630d276b9abfdd4bef2f328794c4e6
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>