msm: vidc: limit only OCMEM vote to lowest vector

Previously the driver was preventing any bus vote from going down to 0
as part of vote_buses().  This was to prevent the ocmem core from
accidentally resetting.  Since this is a ocmem specific issue, we can
allow DDR votes to go to 0, which affords us some minor power saving.

CRs-Fixed: 713172
Change-Id: I63396710236cb5dc4f983801eda25ab92e97ac1a
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>