drm/msm/mdp5: Enable DSI connector in msm drm driver

This change adds the support in mdp5 kms driver for single
and dual DSI. Dual DSI case depends on the framework API
and sequence change to support dual data path.

v1: Initial change
v2: Address Rob Clark's comment
- Separate command mode encoder to a new file mdp5_cmd_encoder.c
- Rebase to not depend on msm_drm_sub_dev change

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