msm: kgsl: Zap performance counters across context switches

Performance counter values need not be retained across contexts unless
specifically requested for debug. Zap the counters by initialising
perfcounter SRAM with 0's using GPU_RBBM_PERFCTR_SRAM_INIT_CMD.

Add pm4 packets during context switches and add a KMD postamble packet to
clear the counters during preemption. Do not enable perfcounter save and
restore unless requested.

Change-Id: I371779ce659c07a1cc664327f5ecdcf0374201d8
Signed-off-by: Mohammed Mirza Mandayappurath Manzoor <quic_mmandaya@quicinc.com>