qcacmn: Use atomic allocation for all scheduler context allocations

In low memory environments using GFP_KERNEL flag may cause
scheduler thread to sleep. Scheduler thread callback handlers
are expected to be atomic in nature to ensure timely execution
of different commands.

Change-Id: Iee3eafbc00a3afea0687ba67b3041ec0816094cc
CRs-Fixed: 2232553