msm: kgsl: Don't attempt to flush an invalid useraddr

In a cache operation the offset is added to a buffer user address
before it is checked for NULL which could result in a cache operation
on an invalid address.

Change-Id: Ic0dedbad3fbc71f830fc5ce79ab1dd6dee5e1d04
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>