drm/vc4: Ensure zpos is always initialised

The compiler is warning that default_zpos can be used
uninitialised as there is no default case to catch all plane
types.
No other plane types should ever be presented to vc4_fkms_plane_init,
but add a default case regardless.

Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>