platform: msm: add support for external display module

Add support for the external display module, which is a utility
to manage display interface and audio codec interactions
associated with the cable connect/disconnect events of the
display interfaces. The helper class is responsible for
routing operations called by the audio codec to a particular
display, and for creating and updating the audio switch nodes.

This implementation is based on a snapshot of the external
display module as of this commit 104eef6e1fe8d4d2 ("platform:
msm: Remove unused local count variable") on kernel-4.19.

Change-Id: I919adf3fda110ecba8d7054a1385e2cbdaca2467
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>