drm/vc4: plane: Move planes creation to its own function

The planes so far were created as part of the CRTC binding code with
each planes created associated only to one CRTC. However, the hardware
in the vc4 doesn't really have such constraint and can be used with any
CRTC.

In order to rework this, let's first move the overlay and cursor planes
creation to a function of its own.

Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/a378ea56214179f1f25fcd36ecc69511edd1e790.1590594512.git-series.maxime@cerno.tech