msm: vidc: Check idle status before power collapse

Idle status indicates whether the video hw core has
completed its task and is ready for power collapse.
Both WFI and IDLE should be set to power collapse the
core.

CRs-Fixed: 2384822
Change-Id: I630c342e8337141552377040727ecbaf143fe661
Signed-off-by: Chinmay Sawarkar <chinmays@codeaurora.org>