msm: kgsl: Fix context initialization sequence

Re-order the preemption context initialization in order to
avoid NULL pointer dereference if preemption_context_init
fails. Also, correctly handle the failure by removing
the put/kfree as the context will be destroyed in the detach
sequence itself.

Change-Id: Ic8b373da040d39bc412ed54a21f067bd51685f6a
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>