drm/msm/sde: add prepare_commit callback to sde encoder

Allow the sde encoder to react to prepare_commit at the
beginning of an atomic commit sequence to support features like
autorefresh which need to be disabled before any other register
writes take place.

Change-Id: I2891eeba0cc9cdb6f3c6a5c2cc3542d74fa3ba1e
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>