drm/msm: convert to drm_bridge

Drop the msm_connector base class, and special calls to base class
methods from the encoder, and use instead drm_bridge.  This allows for a
cleaner division between the hdmi (and in future dsi) blocks, from the
mdp block.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>