msm: vidc: Don't accidentally unvote for buses when vote_bus is called

The client occasionally calls vote_bus with load of 0, which actually is
translated to an unvote_bus.  Since this isn't likely what the client
intended, vote for the lowest bus vector.

As a side effect of the above bug, ocmem_free occasionally failed as the
driver accidentally unvoted for ocmem noc bus, which reset the ocmem core.

CRs-Fixed: 649903
Change-Id: I8504d5f660b881d757245db3b33521443bc3073e
Signed-off-by: Deva Ramasubramanian <dramasub@codeaurora.org>