drm/msm/dsi: Use one connector for dual DSI mode

Current DSI driver uses two connectors for dual DSI case even
though we only have one panel. Fix this by implementing one
connector/bridge for dual DSI use case. Use master DSI
controllers to register one connector/bridge.

Changes in v3:
- None

Reviewed-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Chandan Uddaraju <chandanu@codeaurora.org>
[seanpaul removed unused local var causing a build warning]
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>