msm: vidc: reduce clk frquency before disable

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>