drm/msm/mdp5: Bring back pipe_lock to mdp5_plane struct

We'd previously moved the pipe_lock spinlock to the hwpipe struct. Bring
it back to mdp5_plane. We will need this because an mdp5_plane in the
future could comprise of 2 hw pipes. It makes more sense to have a single
lock to protect the registers for the hw pipes used by a plane, rather
than trying to take individual locks per hwpipe when committing a
configuration.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>