cpufreq: schedutil: Fix race condition in computing hispeed_util

hispeed_util can be changed in the context of the store function and
scheduler notifications. So, we need to grab the update_lock (used to
protect members of sg_policy) before changing it to avoid any race
conditions.

Change-Id: I0c6336bab0ec265d900d2e16df1fe95824d7b2e8
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>