drm/msm/dpu: Correct dpu encoder spinlock initialization

dpu encoder spinlock should be initialized during dpu encoder
init instead of dpu encoder setup which is part of modeset init.

Signed-off-by: Shubhashree Dhar <dhar@codeaurora.org>
[seanpaul resolved conflict in old init removal and revised the commit message]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1561357632-15361-1-git-send-email-dhar@codeaurora.org