msm: kgsl: Check for gmu prealloc failure only in case of prealloc request

Check for gmu prealloc failure only in case of prealloc request to avoid
use of uninitialized ret variable.

Change-Id: Ie0718f17fb9517e3e7659c277ff6f2fc3ef07f53
Signed-off-by: Deepak Kumar <dkumar@codeaurora.org>