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: Amit Kushwaha <[email protected]>