drm/msm: update use of drm_bridge_attach in dp and dsi drivers

In 4.14 the API for drm_bridge_attach has been changed to include
a previous bridge parameter.  This change updates the DP and DSI
drivers ported from 4.9 to be compatible with this new API.

Change-Id: Ie25e2747cea9a9f4c48f34fba595c992fd1ff51d
Signed-off-by: Steve Cohen <cohens@codeaurora.org>