drm/vc4: crtc: Rework the encoder retrieval code (again)

It turns out the encoder retrieval code, in addition to being
unnecessarily complicated, has a bug when only the planes and crtcs are
affected by a given atomic commit.

Indeed, in such a case, either drm_atomic_get_old_connector_state or
drm_atomic_get_new_connector_state will return NULL and thus our encoder
retrieval code will not match on anything.

We can however simplify the code by using drm_for_each_encoder_mask, the
drm_crtc_state storing the encoders a given CRTC is connected to
directly and without relying on any other state.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>