msm: mdss: add external display class helper

Implement the external display class helper as 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, updating the hdmi switch node, and updating the
audio switch node.

CRs-Fixed: 1009284
Change-Id: Ie8d1006d3f11091a861733485cb67939ad47fdfe
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>