drm/i915: Add .atomic_{get, set}_property() entrypoints to planes

When we flip on the DRIVER_ATOMIC bit, the DRM core will start calling
this entrypoint to set and lookup driver-specific plane property values,
rather than maintaining a shadow copy in object->properties.

Note that although we add these functions to the plane vtable, they will
not yet be called.  Future patches that switch our .set_property()
handler and/or enable full atomic functionality are required before
these code paths will be executed.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>