msm: kgsl: Take devfreq event lock for ddr qos vote

This lock protects against any concurrent governor re-assignments
through sysfs. Furthermore, such re-assignment can lead to another
issue i.e. say someone assigns performance governor to this device
from sysfs. Then there can be a mismatch in what kgsl intends to
vote, and what actually gets voted. Therefore, fix both the min and
max frequencies for this devfreq device at the desired qos level so
that the intended vote is propagated regardless of the governor.

Change-Id: Ibc3b0ef1f585dae311aab028d8c2bc9f1a53b4b8
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>