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 3c1b6b9787c7ee06 ("msm: mdss:
remove HDMI connected flags") on kernel-4.4.

CRs-Fixed: 2020938
Change-Id: Ifcfb0ef6e08d979fb3a6dfe8d878e24283a45389
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>