msm: vidc: Wait for Arm9 to go to WFI before power collapsing

Wait for Arm9 to go into Wait For Interrupt (WFI) state before
power collapsing venus. This is to make sure Venus CPU is idle
when it is power collapsed.

Change-Id: I47f7f8661fbb71453416dc2340b9e85e856d3539
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>