drm/atomic: Add __drm_atomic_helper_plane_reset

There are a lot of drivers that subclass drm_plane_state, all of them
duplicate the code that links together the plane with plane_state.

On top of that, drivers that enable core properties also have to
duplicate the code for initializing the properties to their default
values, which in all cases are the same as the defaults from core.

Change since v1:
- Make it consistent with the other helpers and require that both
  plane and state not be NULL, suggested by Boris Brezillon and
  Philipp Zabel.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Alexandru Gheorghe <alexandru-cosmin.gheorghe@arm.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20180804161530.12275-2-alexandru-cosmin.gheorghe@arm.com