drm/exynos: soft reset mixer before reconfigure after power-on

Mixer soft reset is a recommended step before reconfiguring
the mixer after power on. Mixer looses the previous state of
DMAs if soft reset. This is the recommendation from the
hardware team.
Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_mixer.c

@@ -1085,6 +1085,8 @@ static void mixer_poweron(struct exynos_drm_manager *mgr)
 	ctx->powered = true;
 	mutex_unlock(&ctx->mixer_mutex);
 
+	mixer_reg_writemask(res, MXR_STATUS, ~0, MXR_STATUS_SOFT_RESET);
+
 	mixer_reg_write(res, MXR_INT_EN, ctx->int_en);
 	mixer_win_reset(ctx);