msm: kgsl: BUG_ON only if address is not an error

_gpu_find_svm() makes a BUG_ON if the returned address is
greater than ULONG_MAX.

But in 32bit kernel, Error conditions would also make the
comparison to be true. Because it compares address or
error in unit64_t with ULONG_MAX which is in 32bit.

Check whether returned address is an error before making the BUG_ON.

Change-Id: I482b330db3e06a1bee31dd6931faf239a61f9ab8
Signed-off-by: Rajesh Kemisetti <rajeshk@codeaurora.org>