ASoC: msm: Add pan/scale & downmix control as platform mixer controls

During soundcard deregistration, free_chmap() is called as part of
pcm_free. This only frees downmix kcontrol as pcm has reference
only to downmix kcontrol followed by freeing of pcm pointer. The
other two kcontrol are freed as part of snd_ctl_remove(). However,
while freeing other two kcontrols, pcm_usr_ctl_private_free()
accesses pcm->streams which are already freed causing crash.
Fix is to add pan/scale & downmix controls as platform controls
instead of user control.

Change-Id: Ib2890122d079201e02c4b7839694d2e7618037b7
Signed-off-by: Varun Balaraj <varunb@codeaurora.org>