drm/vc4: dpi: Embed DRM structures into the private structure

The VC4 DPI driver private structure contains only a pointer to the
encoder it implements. This makes the overall structure somewhat
inconsistent with the rest of the driver, and complicates its
initialisation without any apparent gain.

Let's embed the drm_encoder structure (through the vc4_encoder one) into
struct vc4_dpi to fix both issues.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>