msm: kgsl: Add a sysfs node to control performance counter reads

Currently performance counters are global and can be read by anyone. Change
the behaviour to disable reading global counters as default and add a sysfs
node to enable/disable reads.

Change-Id: Ic3785acd9bd7425c2a844ed103d7b870d9f80adf
Signed-off-by: Mohammed Mirza Mandayappurath Manzoor <quic_mmandaya@quicinc.com>
Signed-off-by: Pankaj Gupta <quic_gpankaj@quicinc.com>
Signed-off-by: Harshitha Sai Neelati <quic_hsaineel@quicinc.com>