drm/msm/dsi: remove msm_dsi::encoder

There is no need anymore to stop the drm_encoder instance in struct
msm_dsi. Remove corresponding field.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/561697/
Link: https://lore.kernel.org/r/20231009205727.2781802-5-dmitry.baryshkov@linaro.org