drm: Add drm_bridge

This patch adds the notion of a drm_bridge. A bridge is a chained
device which hangs off an encoder. The drm driver using the bridge
should provide the association between encoder and bridge. Once a
bridge is associated with an encoder, it will participate in mode
set, and dpms (via the enable/disable hooks).

Signed-off-by: Sean Paul <seanpaul@chromium.org>
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>