rpmsg-kdrv: add support for DISPLAY class

This patch adds support for DISPLAY class of
virtual devices which can be used by DRM framework
to access display-specific services exposed by
remote cores.

The following APIs are exposed by the virtual backend:
* ready: Query remote side initialised display
  H/W and is ready to accept frames.

  Uses REQUEST / RESPONSE protocol

* get_res_info: Get information on available display
  channels / resources (virtual planes , virtual CRTCs etc)
  exposed by the remote server

  Uses REQUEST / RESPONSE protocol

* commit: Send a commit request (atomic bundle of all
  plane updates for a virtual CRTC) to be applied on the
  H/W.

  Uses REQUEST / RESPONSE protocol

  RESPONSE indicates whether the request could be
  processed or not.

  The actual indications of H/W updates and buffer
  reusability are sent as events from server to client
  which can be handled by registering .commit_done()
  and .buffer_done() callbacks

Tested-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Subhajit Paul <subhajit_paul@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>