drm/msm/mdp5: use drm_plane_state for storing alpha value

Use drm_plane_state's 'alpha' field rather than adding extra 'alpha'
field to mdp5_plane_state.

Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://lore.kernel.org/r/20210525131316.3117809-3-dmitry.baryshkov@linaro.org


Signed-off-by: Rob Clark <robdclark@chromium.org>