vidc: Correct start and stop sequence.

Video core stop should be called only if start is successful.

Original commit: https://www.codeaurora.org/cgit/quic/la/kernel/msm/commit/drivers/video/msm/vidc/common/enc/venc.c?h=jb_vanilla&id=204d123b3b46fcd5d491d95525b42b61e783309c