msm: kgsl: Reserve a context ID slot but don't populate immediately

When creating a context allocate an ID but don't populate the slot
with the context pointer until we are done setup up the rest of the
process. This avoids a race if somebody tries to free the same
identifier before the create operation is complete.

Change-Id: Ic0dedbadca5b4cc4ce567afad48a33078b549439
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Dumpeti Sathish Kumar <sathyanov14@codeaurora.org>