msm: kgsl: Add kernel API support for limits mitigation

Add a kernel interface to limit GPU power level during
high-risk low voltage or high current conditions of the
battery.

kgsl_pwr_limits_add is called by every client to add a node
to the limits list. The client can then call other APIs such as
kgsl_pwr_limits_set_freq or kgsl_pwr_limits_set_default to set limits.
Once the client is done with limits it should delete the limits node
by calling kgsl_pwr_limits_del.

Limits can be set from both kernel APIs and sysfs nodes.
Make sure all votes are considered before setting the final limits.

Change-Id: I826e7265f67051aecb6f6e191e338930401a6f8b
Signed-off-by: Suman Tatiraju <sumant@codeaurora.org>