drm/msm/dsi-staging: add external bridge support

Add the external bridge support in DSI driver. This
enables the dsi-staging driver to work with external
bridges who can translate DSI to other output formats
such as HDMI/DP.

CRs-Fixed: 2140653
Change-Id: I1af0404f9eb2c4e37a0998e157ec2c56b972f155
Signed-off-by: Ray Zhang <rayz@codeaurora.org>