msm: ipa: protect q6_proxy_clk_vote

Add a mutex to protect q6_proxy_clk_vote.
The proxy vote/unvote may be called from different contexts
which can result in a a race condition.

Change-Id: I402090be5ff35897ed61499a13bf52eb60a1f5cf
CRs-Fixed: 2094724
Acked-by: Ady Abraham <adya@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>