drm/msm/dsi: Modify dsi manager bridge ops to work with external bridges

The dsi bridge ops call drm_panel functions to set up the connected
drm_panel. Add checks to make sure these aren't called when we're
connected to an external bridge.

Signed-off-by: Archit Taneja <architt@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>