drm/msm/mdp5: release hwpipe(s) for unused planes

Otherwise, if userspace doesn't re-use a given plane, it's hwpipe(s)
could stay permanently assigned.

Signed-off-by: Rob Clark <robdclark@gmail.com>
[Archit: removed right hwpipe stuff for 4.9 kernel]
Signed-off-by: Archit Taneja <architt@codeaurora.org>