drm/msm/dp: Remove encoder pointer from struct msm_dp

We don't need to stash the encoder here. Instead we can simply pass it
around as an argument.

Cc: Kuogee Hsieh <quic_khsieh@quicinc.com>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/491187/
Link: https://lore.kernel.org/r/20220625002811.3225344-1-swboyd@chromium.org
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>