drm/bridge: make bridge registration independent of drm flow

Currently, third party bridge drivers(ptn3460) are dependent
on the corresponding encoder driver init, since bridge driver
needs a drm_device pointer to finish drm initializations.
The encoder driver passes the drm_device pointer to the
bridge driver. Because of this dependency, third party drivers
like ptn3460 doesn't adhere to the driver model.

In this patch, we reframe the bridge registration framework
so that bridge initialization is split into 2 steps, and
bridge registration happens independent of drm flow:
--Step 1: gather all the bridge settings independent of drm and
	  add the bridge onto a global list of bridges.
--Step 2: when the encoder driver is probed, call drm_bridge_attach
	  for the corresponding bridge so that the bridge receives
	  drm_device pointer and continues with connector and other
	  drm initializations.

The old set of bridge helpers are removed, and a set of new helpers
are added to accomplish the 2 step initializ...