drm/msm/dp: add audio support for Display Port on MSM

Introduce audio support for Display Port on MSM chipsets.
This change integrates DP audio sub-module with the main
Display Port platform driver.

In addition, this change leverages hdmi_codec_ops to expose
the operations to the audio driver.

Changes in v2: fix up a compilation issue on drm-next branch
Changes in v3: none
Changes in v4: none
Changes in v5: none
Changes in v6: rebase on top of latest patchset of dependency

Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>