msm: vidc: Unvote for bus BW after unloading FW

Driver should unvote for bus bandwidth after all
transactions are completed. Unvoting after unloading
firmware makes sure there are no outstanding transactions.
Without this change, stability of video playback might
be impacted when video is played back to back.

CRs-fixed: 419402
Change-Id: I821f941183fc24ba65d6f17b3f39354e0d77b4bf
Signed-off-by: Vinay Kalia <vkalia@codeaurora.org>