drm/msm: Initial add DSI connector support

This change adds the DSI connector support in msm drm driver.

v1: Initial change
v2:
- Address comments from Archit + minor clean-ups
- Rebase to not depend on msm_drm_sub_dev change [Rob's comment]
v3: Fix issues when initialization is failed

Signed-off-by: Hai Li <hali@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>