ASoC: msm: implement drain command in compress platform driver

Currently, at the end of compress audio playback, session is
terminated without ensuring compressed audio stream is completely
rendered. Use ioctl drain to drain out all the data before EOS is
signalled and session is closed. This is to make sure all the
data is played out by DSP.

CRs-Fixed: 390897
Change-Id: I3e39b210a8924180de9eb4453c6f6f6435ded0ee
Signed-off-by: Krishnankutty Kolathappilly <kkolat@codeaurora.org>

Conflicts:
	sound/core/pcm_native.c
	sound/soc/msm/msm-compr-q6.c