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 <[email protected]>
Signed-off-by: Pranav Patel <[email protected]>