drm/msm/dpu: add display port support in DPU

Add display port support in DPU by creating hooks
for DP encoder enumeration and encoder mode
initialization.

changes in v2:
	- rebase on [2] (Sean Paul)
	- remove unwanted error checks and
	  switch cases (Jordan Crouse)

[1] https://lwn.net/Articles/768265/
[2] https://lkml.org/lkml/2018/11/17/87



changes in V3:
-- Moved this change as part of the DP driver changes.
-- Addressed compilation issues on the latest code base.

Changes in v6:
-- Fix checkpatch.pl warning

Changes in v7: Remove depends-on tag from commit message.

Changes in v8: None

Changes in v9: None

Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>
Signed-off-by: Vara Reddy <varar@codeaurora.org>
Signed-off-by: Tanmay Shah <tanmay@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>