drm/msm/mdp5: use drmm-managed allocation for mdp5_encoder

Change struct mdp5_encoder allocation to use drmm_encoder_alloc(). This
removes the need to perform any actions on encoder destruction.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/546164/
Link: https://lore.kernel.org/r/20230708010407.3871346-10-dmitry.baryshkov@linaro.org