msm: kgsl: Bail out when two threads add sparse bindings

If we find that a different thread has already added bindings at the
same offset we wanted to add to the sparse object, don't get stuck in
an infinite loop, and return with an error.

Change-Id: I6b17c91eccb14c07e13cae24135dfe7b13f3301d
Signed-off-by: Lynus Vaz <lvaz@codeaurora.org>