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

The VC4 VEC driver private structure contains only a pointer to the
encoder and connector 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) and
drm_connector into struct vc4_vec to fix both issues.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>