drm/i915/display: Explicitly cleanup initial_plane_config

I am about to stuff more objects into the plane_config and would like to
have it clean up after itself. Move the current framebuffer release into
a common function so it can be extended with the new object with
relative ease.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200204094801.877288-1-chris@chris-wilson.co.uk