ASoC: don't dereference NULL pcm_{new,free}

Not all platform drivers have pcm_{new,free} callbacks. Seen with a
"snd-soc-dummy" codec from sound/soc/rockchip/rk3399_gru_sound.c.

Fixes: 99b04f4c ("ASoC: add Component level pcm_new/pcm_free")
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>