drm/msm: dpu: Don't reset dpu_enc->cur_master on .disable()

cur_master in dpu_encoder is assigned at modeset and cleared on
.disable(). Unfortunately dpms (or enable/disable) does not guarantee a
modeset, so cur_master is NULL when we try to re-enable it.

This patch moves the NULL assignment to setup_display where it will be
re-assigned later in the function.

Tested-by: Bruce Wang <bzwang@chromium.org>
Reviewed-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>

Signed-off-by: Rob Clark <robdclark@gmail.com>