soc: qcom: hgsl: fix potential UAF

potential UAF due to race condition in hgsl_ioctl_ctxt_create
and hgsl_ioctl_ctxt_destroy, assign ctxt to hgsl->contexts[]
after the context creation is fully complete.

Change-Id: I07ee64579fa4c9e494b1c5f21053baaa6a027ec2
Signed-off-by: Hui Li <quic_hul@quicinc.com>