msm: vidc: Reduce clock frequency before disabling

The change ensures that the clock frequency is reduced to min
before we disable it. With just clk_disable, rpm still votes
for high voltage on CX as venus vote is not removed. To unvote,
the frequency is set to min.

CRs-Fixed: 574365
Change-Id: I8e0515b9816f8c451d545443573afaba611aafe7
Signed-off-by: Vikash Garodia <vgarodia@codeaurora.org>