msm: kgsl: Idle the core before changing the gpu clock.

Idle the core before changing the gpu clock as instability
is caused on some platforms on changing the clock freq when
core is busy.

CRs-fixed: 328249
Signed-off-by: Kedar Joshi <kjoshi@codeaurora.org>
(cherry picked from commit c11d098f53dd1fb07ea03475510c29c39b9671ab)

Change-Id: I11fc58128560b131f60a84fc94b8bc48134a9eaa
Signed-off-by: Srivalli Oguri <oguri@codeaurora.org>